The phosphor CaTiO₃:Pr³⁺ was synthesized via a solid-state reaction in combination with a subsequent annealing under flowing NH₃. Comparatively large off-center displacements of Ti in the TiO₆ octahedra were confirmed for as-synthesized CaTiO₃:Pr³ by XANES. Raman spectroscopy showed that the local crystal structure becomes highly symmetric when the powders are ammonolyzed at 400 °C. Rietveld refinement of powder X-ray diffraction data revealed that the samples ammonolyzed at 400 °C have the smallest lattice strain and at the same time the largest average Ti-O-Ti angles were obtained. The samples ammonolyzed at 400 °C also showed the smallest mass loss during the thermal re-oxidation in thermogravimetric analysis (TGA). Enhanced photolumincescence brightness and an improved decay curve as well as the highest reflectance were obtained for the samples ammonolyzed at 400 °C. The improved photoluminescence and afterglow by NH₃ treatment are explained as a result of the reduced concentration of oxygen excesses with simultaneous relaxation of the lattice strain.

The emission spectra of Sm²⁺ doped in BaFBr and SrFBr hosts were measured at 10 K from ambient pressure to 8 GPa. The crystal field energy levels determined from the emission spectra were used to extract the free ion parameters (F_k and ζ ) and crystal field parameters (B_q^k). The variation of F_k and ζ as a function of pressure was studied systematically and was discussed in relation to the central field and symmetry restricted covalency models. The change of the spin orbit coupling parameter (ζ) with pressure for SrFBr:Sm²⁺ showed very different behavior than in other matlockite hosts. Moreover the variation of B_q^k under pressure was studied. The pressure dependence of the B_q^k was described quantitatively using the Superposition Model (SM) with the help of structural parameters as a function of pressure, obtained from periodic DFT calculations. The validity of the SM was tested for Sm²⁺ in BaFBr and SrFBr. It is shown that this model does not apply to SrFBr, in contrast to other matlockite host materials.

Four novel bimetallic borohydrides have been discovered, K₂M(BH₄)₄ (M = Mg or Mn), K₃Mg(BH₄)₅, and KMn(BH₄)₃, and are carefully investigated structurally as well as regarding their decomposition reaction mechanism by means of in situ synchrotron radiation powder X-ray diffraction (SR-PXD), vibrational spectroscopies (Raman and IR), thermal analysis (TGA and DTA), and ab initio density functional theory (DFT) calculations. Mechano-chemical synthesis (ball-milling) using the reactants KBH₄, α-Mg(BH₄)₂, and α-Mn(BH₄)₂ ensures chlorine-free reaction products. A detailed structural analysis reveals significant similarities as well as surprising differences among the two isomorphs K₂M(BH₄)₄, most importantly concerning the extent to which the complex anion [M(BH₄)₄]^2– is isolated in the structure. Anisotropic thermal expansion and an increase in symmetry at high temperatures in K₃Mg(BH₄)₅ is ascribed to the motion of BH₄ groups inducing hydrogen repulsive effects, and the dynamics of K₃Mg(BH₄)₅ are investigated. Decomposition in the manganese system proceeds via the formation of KMn(BH₄)₃, the first perovkite type borohydride reported to date.

Hydrogen production from waste feedstocks using supercritical water gasification (SCWG) is a promising approach towards cleaner fuel production and a solution for hard to treat wastes. In this study, the catalytic co-gasification of starch and catechol as models of carbohydrates and phenol compounds was investigated in a batch reactor at 28 MPa, 400–500 °C, from 10 to 30 min. The effects of reaction conditions, and the addition of calcium oxide (CaO) as a carbon dioxide (CO₂) sorbent and TiO₂ as catalyst on the gas yields and product distribution were investigated. Employing TiO₂ as a catalyst alone had no significant effect on the H₂ yield but when combined with CaO increased the hydrogen yield by 35% and promoted higher total organic carbon (TOC) reduction efficiencies. The process liquid effluent was characterized using GC–MS, with the results showing that the major non-polar components were phenol, substituted phenols, and cresols. An overall reaction scheme is provided.

Red emitting CaTiO₃:Pr phosphors with a nominal composition of Ca_0.998+xPr_0.002TiO_3+δ (0.02≤x≤0.04) were prepared by solid state reactions with different thermal post treatments and characterized by X-ray diffraction, transmission electron microscopy and photoluminescence. The Ca excess exhibited complete solubility up to 4% in the samples treated at 1400 °C but segregation in the form of Ruddlesden-Popper phases (Ca₃Ti₂O₇ - Ca₄Ti₃O₁₀) was observed in samples prepared at 1500 °C. The increase in temperature for stoichiometric samples showed a monotonic increase of decay time due to the reduction of non-radiative recombination defects. It was found that the Ca excess favored the formation of oxygen vacancies which are known to act as trap. In the samples treated at 1400 °C, 3% of Ca excess showed to be the best concentration to increase the decay time of persistent luminescence. For the samples treated at 1500 °C, the segregation of Ruddlesden-Popper phases left a constant amount of Ca soluble in all the CaTiO₃ samples. This constant concentration of Ca caused the same density of defects and, consequently, the same decay time in all samples.

The crystal chemistry of the barium fluoride chloride system is studied both experimentally and theoretically. Different synthetic approaches yield nanocrystalline materials as well as large single crystals. The crystalline phases identified so far are BaFCl, Ba₁₂F₁₉Cl₅ and Ba₇F₁₂Cl₂ (in two modifications) and compared with analogous compounds. It is demonstrated that the compound Ba₂F₃Cl reported by Fessenden and Lewin 50 years ago corresponds to Ba₇F₁₂Cl₂. The phase diagram of the BaCl₂ – BaF₂ system is reinvestigated for fluoride mole fractions between 0.5 and 1. The peritectic formation of Ba₁₂F₁₉Cl₅ is observed. Periodic DFT calculations are performed for all structures in this system, including a hypothetical structure for Ba₂F₃Cl, based on the experimental structure of Ba₂H₃Cl. The energy of formation of the different barium fluoride chloride compounds from BaCl₂ and BaF₂ (normalized for one barium atom per formula unit), as calculated by DFT at 0K, is within only about ± 15 kJ/mol. Comparison with recent experimental results on calcium and strontium hydride chloride (bromide) compounds, suggest the possibility of a mutual exclusion between the M₂X₃Y and M₇X₁₂Y₂ (M = Ca, Sr, Ba, Pb, X = H, F, Y = Cl,Br) structures. The single crystal structure of PbFBr is also reported.

To study the reorientational motion of BH₄ groups in β and γ phases of Mg(BH₄)₂ and in α and β phases of Ca(BH₄)₂, we have performed nuclear magnetic resonance (NMR) measurements of the ¹H and ¹¹B spin–lattice relaxation rates in these compounds over wide ranges of temperature and resonance frequency. It is found that at low temperatures the reorientational motion in β phases of Mg(BH₄)₂ and Ca(BH₄)₂ is considerably faster than in other studied phases of these alkaline-earth borohydrides. The behavior of the measured spin–lattice relaxation rates in both β phases can be satisfactorily described in terms of a Gaussian distribution of activation energies E_a with the average E_a values of 138 meV for β-Mg(BH₄)₂and 116 meV for β-Ca(BH₄)₂. The α phase of Ca(BH₄)₂ is characterized by the activation energy of 286 ± 7 meV. For the novel porous γ phase of Mg(BH₄)₂, the main reorientational process responsible for the observed spin–lattice relaxation rate maximum can be described by the activation energy of 276 ± 5 meV. The barriers for reorientational motion in different phases of alkaline-earth borohydrides are discussed on the basis of changes in the local environment of BH₄ groups.

Single crystals of Ce₂RhGa₁₂ have been synthesized using Ga flux and their structure was determined by single-crystal X-ray diffraction. Ce₂RhGa₁₂ crystallizes in the tetragonal space group P4/nbm (No. 125), and is isostructural to Ce₂PdGa₁₂, with Z = 2 and lattice parameters a = 6.0405 Å and c = 15.706 Å. Data were collected at the Swiss Norwegian Beam Line at the European Synchrotron Facility, Grenoble, France. Laue diffraction was carried out to confirm the quality of the single crystal and showed well-defined spots and tetragonal symmetry.

The title compound, an achiral flexible molecule containing a 1,2,3-triazole structure as the acceptor subunit, crystallizes as a single enantiomorph in the space group P2₁2₁2₁. The material exhibits nonlinear optical properties and is capable of second harmonic generation. Thus, the developed molecular scaffold represents an interesting novel type of NLO chromophore.

Highly occupied: A highly porous form of Mg(BH₄)₂ (see picture; Mg green, BH₄ blue, unit cells shown in red) reversibly absorbs H₂, N₂, and CH₂Cl₂. At high pressures, this material transforms into an interpenetrated framework that has 79 % higher density than the other polymorphs. Mg(BH₄)₂ can act as a coordination polymer that has many similarities to metal–organic frameworks.

Barium calcium magnesium fluoride (Ba₂(Ba_xCa_1-x)Mg₄F₁₄, x=0.19-0.26) has been synthesized at 850 °C from precursors prepared by the solution precipitation method. Single crystals with composition of Ba_2.200(2)Ca_0.800(2)Mg₄F₁₄were obtained after prolonged heating. Lattice parameters from single crystal data are a = 12.4203(8) and c = 7.4365(5) Å [tetragonal, space group P4₂/mnm (No. 136)]. They increase with increasing barium concentration within a given stability window. The structure is built of a network of MgF₆ octahedra forming a pyrochlore related channel system and isolated fluorine ions. Within the channels, heavy alkaline earth ions are located. The wide channel is filled with off-center positioned barium ions. The channel with a narrow cross section hosts both ions, Ca²⁺and Ba²⁺. The structure is isotypic with Pb₃Nb₄O₁₂F₂ but has a different coordination around Ba/Ca and Pb, respectively. Doped with ∼1% Eu(II), the compound shows intense blue luminescence under UV activation.

A solid solution of magnesium and manganese borohydrides was studied by in situ synchrotron radiation X-ray powder diffraction and infrared spectroscopy. A combination of thermogravimetry, mass and infrared spectroscopy, and atomic emission spectroscopy were applied to clarify the thermal gas desorption of pure Mn(BH₄)₂ and a solid solution of composition Mg_0.5Mn_0.5(BH₄)₂. Mg_xMn_(1−x)(BH₄)₂ (x = 0–0.8) conserves the trigonal structure of Mn(BH₄)₂ at room temperature. Manganese is dissolved in the hexagonal structure of α-Mg(BH₄)₂, with the upper solubility limit not exceeding 10 mol.% at room temperature. There exists a two-phase region of trigonal and hexagonal borohydrides within the compositional rangex = 0.8–0.9 at room temperature. Infrared spectra show splitting of various vibrational modes, indicating the presence of two cations in the trigonal Mg_xMn_(1−x)(BH₄)₂ solid solutions, as well as the appearance of a second phase, hexagonal α-Mg(BH₄)₂, at higher magnesium contents. All vibrational frequencies are shifted to higher values with increasing magnesium content. The decomposition temperature of the trigonal Mg_xMn_(1−x)(BH₄)₂ (x = 0–0.8) does not vary significantly as a function of the magnesium content (433–453 K). The desorbed gas contains mostly hydrogen and 3–7.5 mol.% diborane B₂H₆, as determined from analyses of the Mn(BH₄)₂ and Mg_0.5Mn_0.5(BH₄)₂ samples. An eutectic relation between α-Mg(BH₄)₂ and LiBH₄ is observed. The solid solution Mg_xMn_(1−x)(BH₄)₂ is a promising material for hydrogen storage as it decomposes at a similar temperature to Mn(BH₄)₂, i.e. at a much lower temperature than pure Mg(BH₄)₂ without significantly losing hydrogen weight capacity thanks to substitution of Mn by Mg up to 80 mol.%. The questions of diborane release and reversibility remain to be addressed.

The synthesis of a novel alkali-metal aluminium borohydride NaAl(BH₄)_xCl_4−x from NaBH₄ and AlCl₃ using a solid state metathesis reaction is described. Structure determination was carried out using synchrotron powder diffraction data and vibrational spectroscopy. An orthorhombic structure (space group Pmn2₁) is formed which contains Na⁺ cations and complex [Al(BH₄,Cl)₄]⁻anions. Due to the high chlorine content (1 ≤ x ≤ 1.43) the hydrogen density of the borohydride is only between 2.3 and 3.5 wt.% H₂ in contrast to the expected 14.6 wt.% for chlorine free NaAl(BH₄)₄. The decomposition of NaAl(BH₄)_xCl_4−x is observed in the target range for desorption at about 90 °C by differential scanning calorimetry (DSC), in situ Raman spectroscopy and synchrotron powder X-ray diffraction. Thermogravimetric analysis (TG) shows extensive mass loss indicating the loss of H₂ and B₂H₆ at about 90 °C followed by extensive weight loss in the form of chloride evaporation.

YMn₂ forms either interstitial YMn₂H_x hydrides for x ≤ 4.5 or a complex YMn₂H₆ hydride when submitted to high hydrogen pressure. These compounds have been studied by inelastic neutron scattering (INS) in order to clarify the different modes of H vibration. The INS spectra of YMn₂H_x hydrides are strongly dependent on the H content. YMn₂H₆ and YMn₂D₆ show broad bands, also observed by Raman and IR spectroscopy, assigned to H–Mn–H (or D) and Mn–H bending and stretching modes. Both ErMn₂D₆ and ErMn_1.8Fe_0.2D₆ show, in addition to the H vibration mode, an intense band at 215 cm⁻¹ which has been attributed to a magnetic excitation of Er³⁺ in view of its momentum transfer dependence.

The structural and vibrational properties of the isostructural compounds Ca₂FeH₆ and Sr₂RuH₆ are determined by periodic DFT calculations and compared with their previously published experimental crystal structures as well as new experimental vibrational data. The analysis of the vibrational data is extended to the whole series of alkaline-earth iron and ruthenium hydrides A₂TH₆ (A = Mg,Ca,Sr; T = Fe, Ru) in order to identify correlations between selected frequencies and the T-H bond length. The bulk moduli of Ca₂FeH₆ and Sr₂RuH₆ have also been determined within DFT. Their calculated values prove to compare well with the experimental values reported for Mg₂FeH₆ and several other compounds of this structure.

The high energy of hydrogen vibrations in solids is the origin of their strong impact on thermodynamic properties. The peculiar structure of complex hydrides amplifies this impact. We shed light on the vibrational properties of three allotropes of Ca(BH₄)₂ using density-functional theory calculations, infrared spectroscopy, and inelastic neutron scattering. We show that the vibrational properties of Ca(BH₄)₂ depend on the specific phase and are hitherto the origin of their differences in stability.

Several new studies of Mg(BH₄)₂ are reported. A 1:1 LiBH₄:Mg(BH₄)₂ mixture was studied by in situ synchrotron X-ray diffraction and reveals an eutectic behavior with the eutectic composition more rich in Mg(BH₄)₂, and the eutectic temperature lower than 456 K. No dual cation compound was observed in this experiment.

New vibrational spectra including INS data have been obtained and are compared with theoretical DFT calculations and recent NMR studies, showing good agreement.

Single crystals of Hydroxyethylammonium L – tartrate monohydrate [HEALT] have been grown by slow evaporation technique using water as a solvent. The structural and vibrational properties of the crystals were studied. Besides these characterizations ab initio quantum chemical calculations have been performed at HF/6-31G (d) level to derive first order hyperpolarizability. It is shown that the first order hyperpolarizability is found to be 14.2 times more than that of urea. The characteristic vibrational frequencies obtained from polarized Raman spectra in different scattering configurations have been assigned based on the complete factor group analysis. Vibrational analysis of IR and Raman reveals that the charge transfer interaction must be responsible for nonlinear optical (NLO) properties of the present system. The UV absorption measurements have also been carried out to confirm the utility of the material for optical applications.

A new potassium scandium borohydride, KSc(BH₄)₄, is presented and characterized by a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. The title compound, KSc(BH₄)₄, forms at ambient conditions in ball milled mixtures of potassium borohydride and ScCl₃ together with a new ternary chloride K₃ScCl₆, which is also structurally characterized. This indicates that the formation of KSc(BH₄)₄ differs from a simple metathesis reaction, and the highest scandium borohydride yield (~31 mol %) can be obtained with a reactant ratio KBH₄:ScCl₃ of 2:1. KSc(BH₄)₄ crystallizes in the orthorhombic crystal system, a = 11.856(5), b = 7.800(3), c = 10.126(6) Å, V = 936.4(8) ų at RT, with the space group symmetry Pnma. KSc(BH₄)₄ has a BaSO₄ type structure where the BH₄ tetrahedra take the oxygen positions. Regarding the packing of cations, K⁺, and complex anions, [Sc(BH₄)₄]⁻, the structure of KSc(BH₄)₄ can be seen as a distorted variant of orthorhombic neptunium, Np, metal. Thermal expansion of KSc(BH₄)₄ in the temperature range RT to 405 K is anisotropic, and the lattice parameter b shows strong nonlinearity upon approaching the melting temperature. The vibrational and NMR spectra are consistent with the structural model, and previous investigations of the related compounds ASc(BH₄)₄ with A = Li, Na. KSc(BH₄)₄ is stable from RT up to ~405 K, where the compound melts and then releases hydrogen in two rapid steps approximately at 460−500 K and 510−590 K. The hydrogen release involves the formation of KBH₄, which reacts with K₃ScCl₆ and forms a solid solution, K(BH₄)_1−xCl_x. The ternary potassium scandium chloride K₃ScCl₆ observed in all samples has a monoclinic structure at room temperature, P2₁/a, a = 12.729(3), b = 7.367(2), c = 12.825(3) Å, β = 109.22(2)°, V = 1135.6(4) ų, which is isostructural to K₃MoCl₆. The monoclinic polymorph transforms to cubic at 635 K, a = 10.694 Å (based on diffraction data measured at 769 K), which is isostructural to the high temperature phase of K₃YCl₆.

A combination of in situ synchrotron powder diffraction, energy minimization (DFT), and Raman and infrared spectroscopy confirmed porous interpenetrated 3D-framework structures of recently discovered alkali-metal−zinc borohydrides, AZn₂(BH₄)₅ (A = Li, Na). In the less zinc rich NaZn(BH₄)₃ the 3D-framework structural model has been confirmed but with a slightly modified description giving an isolated triangular anion, [Zn(BH₄)₃]⁻, rather than a 1D anionic chain, {[Zn(BH₄)₃]_n}ⁿ⁻. Another polymorph of NaZn(BH₄)₃, isostructural to a new compound, LiZn(BH₄)₃, is proposed by energy minimization. Both compounds, the new NaZn(BH₄)₃ polymorph and LiZn(BH₄)₃, are, however, not observed experimentally at ambient pressure and in the temperature range of 100−400 K. The alkali-metal−zinc borohydride NaZn(BH₄)₃ containing the triangular anion [Zn(BH₄)₃]⁻ is an equivalent of recently characterized alkali-metal−scandium borohydrides NaSc(BH₄)₄ and LiSc(BH₄)₄ based on the tetrahedral [Sc(BH₄)₄]⁻ complex anion.

The crystal structure of recently reported Ba,F,Cl nanorods is shown to correspond to the structure of Ba₇F₁₂Cl₂ (see picture), which can be prepared by several growth techniques.

The mechanochemical reaction of LiBH₄ with MnCl₂ produces the neutral complex Mn(BH₄)₂. Thermal desorption studies show that the mechanochemical reaction of NaBH₄ with MnCl₂produces a different species, apparently Na₂Mn(BH₄)₄, that undergoes dehydrogenation of a much lower weight percent H at a ~20 °C higher temperature than the neutral Mn(BH₄)₂. Vibrational spectroscopy also reveals that a complex manganese borohydride(s) in addition to Mn(BH₄)₂ are formed from the mechanochemical reactions. Analysis of the vibrational spectra in conjunction with DFT calculations on a model Mn(BH₄)₄²⁻ complex suggest bidentate binding of the [BH₄]⁻ ligands to the Mn center in the anionic complex. The calculated highest frequencies of the B−H stretching modes (corresponding to the “free” B−H bonds) agree well with the experimental frequencies and support the presence of this structural feature.

The decomposition pathway in LiBH₄−MgH₂ reactive hydride composites was investigated systematically as a function of pressure and temperature. Individual decomposition of MgH2 and LiBH4 is observed at higher temperatures and low pressures (T ≥ 450 °C and p(H₂) ≤ 3 bar), whereas simultaneous desorption of H₂ from LiBH₄ and formation of MgB₂ was observed at 400 °C and a hydrogen backpressure of p(H₂) = 5 bar. The simultaneous desorption of H₂ from LiBH₄ and MgH₂ without intermediate formation of metallic Mg could not be observed. In situ X-ray diffraction (XRD) and infrared (IR) spectroscopy reveal the present crystalline and amorphous phases.

The effect on crystal structure and vibrational frequencies of physical pressure in BaFCl and chemical pressure in Ba_1−xSr_xFCl solid solutions is studied using periodic density-functional theory (DFT) calculations performed within the local-density approximation (LDA) and the generalized gradient approximation (GGA). These results are compared with previously published experimental data for BaFCl in conjunction with new experimental data for Ba_1−xSr_xFCl and show overall a good agreement with experiment. The GGA method outperforms the LDA method for the description of BaFCl under pressure. However, the two DFT methods perform equally well for the description of the solid solutions, which have been studied within the virtual-crystal approximation. They also give consistent values of the energy of formation of Ba_1−xSr_xFCl, which can be correlated with the experimentally observed melting points. The comparison of the calculated mode Grüneisen parameters shows that, for the investigated systems, the effect of the chemical pressure and that of the physical pressure are not identical.

The new double-cation Al-Li-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (~70 °C) combined with a high hydrogen density (17.2 wt %). It was synthesised by high-energy ball milling of AlCl₃ and LiBH₄. The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) Å). The unexpected composition Al₃Li₄(BH₄)₁₃ can be rationalized on the basis of a complex cation [(BH₄)Li₄]³⁺ and a complex anion [Al(BH₄)₄]^-. The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at ~70 °C, forming LiBH₄. The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane.

To study the reorientational motion of BH₄ groups in the low-temperature (α) phase of Mg(BH₄)₂, we have performed nuclear magnetic resonance (NMR) measurements of the ¹H and ¹¹B spin−lattice relaxation rates in this compound over wide ranges of temperature (82−443 K) and resonance frequency (14−90 MHz for ¹H and 14−28 MHz for ¹¹B). It is found that the thermally activated reorientational motion in α-Mg(BH₄)₂ is characterized by a coexistence of at least three jump processes with strongly differing activation energies. Taking into account the anisotropy of the local environment of BH₄ groups in α-Mg(BH₄)₂, these jump processes can be attributed to different types of reorientation. The nearly linear coordination of BH₄ groups by two Mg atoms suggests that the fastest jump process corresponds to the rotation around the 2-fold axis connecting B and two Mg atoms, whereas the slowest process is associated with the rotation around two other 2-fold axes perpendicular to the Mg−B−Mg line.

The pressure/temperature phase diagram of LiAlH₄ has been constructed by using Raman spectroscopy data. In situ high pressure−temperature experiments were carried out using resistively heated diamond anvil cells up to 150 °C and 7 GPa. Room temperature phase transitions of monoclinic α-LiAlH₄ → δ-LiAlH₄ were observed at ~3.2 GPa. As the temperature is increased to ~100 °C, both the α and δ phases transform to β-LiAlH₄ and remain stable up to 5.5 GPa. At temperatures greater than 300 °C, a new γ-LiAlH₄ phase forms. Data of Konovalov (1995) has been used to define the phase boundary between β- and γ-LiAlH₄ phases. We present a pressure−temperature phase diagram of LiAlH₄ based using diamond anvil cells coupled with Raman spectroscopy.

The pressure evolution of RbBH₄ has been characterized by synchrotron powder X-ray diffraction and Raman spectroscopy up to 23 GPa. Diffraction experiments at ambient temperature reveal three phase transitions, at 3.0, 10.4, and 18 GPa (at 2.6, 7.8, and ~20 GPa from Raman data), at which the space group symmetry changes in the order Fm-3m(Z=4) → P4/nmm(2) → C222(2) → I-42m(4). Crystal structures and equations of state are reported for all four phases. The three high-pressure structure types are new in the crystal chemistry of borohydrides. RbBH₄ polymorphs reveal high coordination numbers (CNs) for cation and anion sites, increasing with pressure from 6 to 8, via an intermediate 4 + 4 coordination. Different arrangements of the tetrahedral BH₄ group in the Rb environment define the crystal symmetries of the RbBH₄ polymorphs. The structural evolution in the MBH₄ series is determined by the cation’s size, as it differs drastically for M = Li (CNs = 4, 6), Na (CN = 6), and Rb. The only structure common to the whole MBH₄ family is the cubic one. Its bulk modulus linearly decreases as the ionic radius of M increases, indicating that the compressibility of the material is mainly determined by the repulsive BH₄···BH₄ interactions.

Deuterium−hydrogen exchange in solid α-Mg(BH₄)₂ is demonstrated. Compared to the previously reported exchange reactions in the alkali borohydrides, the temperature at which isotope exchange starts to take place is significantly lower (132 °C vs 200 °C for LiBH₄). The activation energy for the deuterium−hydrogen exchange reaction is estimated to be 51 ± 15 kJ/mol. Almost complete isotope exchange was observed by treating solid Mg(BH₄)₂ for 72 h at 172 °C with 42 bar of D₂. Preliminary experiments indicate that under these conditions Ca(BH₄)₂ also undergoes isotope exchange.

Inorganic borohydrides are actively studied in view of potential hydrogen storage applications. These compounds can be obtained by a variety of reactions ranging from high temperature reactions of the elements to exchange reactions in solution or in solid state. Different approaches will be discussed and compared.

Metal borohydrides are potential materials for solid state hydrogen due to their high gravimetric and volumetric hydrogen densities. Among them, Ca(BH₄)₂ is particularly interesting because of the predicted suitable thermodynamic properties. In this work, we investigate a new synthesis route using high pressure reactive ball milling. Starting from CaH₂ and CaB₆ with a TiCl₃ or TiF₃ as additive, a reaction yield of 19% is obtained after 24 h milling at room temperature and 140 bar H₂. The presence of Ca(BH₄)₂ is confirmed by the presence of the stretching mode of the [BH₄]^- group in the infrared spectra of the as-milled samples. Using in-situ XRD, we observe the recrystallisation of a poorly crystallised Ca(BH₄)₂ phase present after milling. The reversible decomposition/formation of Ca(BH₄)₂ is obtained with higher yield (57%) using higher temperature and TiF₃ as additive but not with TiCl₃ despite its similar electronic structure. The differences observed using different additives and the influence of the anion are discussed.

A new alkaline transition-metal borohydride, NaSc(BH₄)₄, is presented. The compound has been studied using a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. NaSc(BH₄)₄ forms at ambient conditions in ball-milled mixtures of sodium borohydride and ScCl₃. A new ternary chloride Na₃ScCl₆ (P2₁/n, a = 6.7375(3) Å, b = 7.1567(3) Å, c = 9.9316(5) Å, β = 90.491(3)°, V = 478.87(4) ų), isostructural to Na₃TiCl₆, was identified as an additional phase in all samples. This indicates that the formation of NaSc(BH₄)₄ differs from a simple metathesis reaction, and the highest scandium borohydride yield (22 wt %) was obtained with a reactant ratio of ScCl₃/NaBH₄ of 1:2. NaSc(BH₄)₄ crystallizes in the orthorhombic crystal system with the space group symmetry Cmcm (a = 8.170(2) Å, b = 11.875(3) Å, c = 9.018(2) Å, V = 874.9(3) ų). The structure of NaSc(BH₄)₄ consists of isolated homoleptic scandium tetraborohydride anions, [Sc(BH₄)₄]^–, located inside slightly distorted trigonal Na₆ prisms (each second prism is empty, triangular angles of 55.5 and 69.1°). The experimental results show that each Sc³⁺ is tetrahedrally surrounded by four BH₄ tetrahedra with a 12-fold coordination of H to Sc, while Na⁺ is surrounded by six BH₄ tetrahedra in a quite regular octahedral coordination with a (6 + 12)-fold coordination of H to Na. The packing of Na⁺ cations and [Sc(BH₄)₄]^– anions in NaSc(BH₄)₄ is a deformation variant of the hexagonal NiAs structure type. NaSc(BH₄)₄ is stable from RT up to ∼410 K, where the compound melts and then releases hydrogen in two rapidly occurring steps between 440 and 490 K and 495 and 540 K. Thermal expansion of NaSc(BH₄)₄ between RT and 408 K is anisotropic, and lattice parameter b shows strong anomaly close to the melting temperature.

The luminescence of Sm²⁺-doped BaFBr has been measured as a function of temperature and pressure. The ⁷F_J crystal field levels have been identified and the corresponding crystal field parameters evaluated. Temperature dependent lifetime measurements allow to locate the energy of the lowest 4f⁵5d¹ level.

Pressure dependent measurements up to 8 GPa show linear red shifts of the ⁵D_2,1,0 levels. These shifts are about 3 times stronger than the well known ruby pressure shifts and highlight thus the potential use as pressure sensors below 8 GPa.

The historical background and the current activities of the Chemical Society of Geneva are described. Founded 88 years ago from the merger of three student associations, the Society today connects some 190 chemists and biochemists from all professional horizons. The Society's main goal is to promote molecular sciences studies and applications in Geneva. In addition to regularly organizing scientific lectures and visits, it sponsors major scientific seminars and grants annual awards to secondary school pupils and bachelor students.

A brief historical overview of physical chemistry at the University of Geneva as well as a description of the present research activities at the department of physical chemistry are presented.

Specifically labeled NaBD₃H has been synthesized and characterized using X-ray diffraction, NMR, and vibrational spectroscopy. The isotopic purity of the compound, as estimated from NMR spectra, was found to be about 85% with the compound NaBD₂H₂ as the second product. IR spectra confirm the relatively strong intensity of the single B–H stretching mode predicted from DFT calculations. Anharmonic DFT calculations show that for the BD₃H^- ion Fermi resonances with the single B–H stretching mode are very limited, making this mode a promising structural probe for complex borohydrides which can be prepared by metathetical reactions.

New in situ Raman and synchrotron X-ray diffraction data (between 300 and400 K) in conjunction with separate temperature-dependent Raman data(between 7 and 400 K) are presented. The low-frequency Raman spectra showgood agreement with theoretical values obtained previously using periodic DFTcalculations. The temperature-dependent spectra reveal the presence of significantanharmonicity of librational modes neither predicted theoretically nor notedin previous experiments. The splitting of the internal deformation mode ν₂ (of Esymmetry in the free ion) decreases continuously with increasing temperature,but drops abruptly at the first-order orthorhombic to hexagonal phase transitionobserved at 381 K. The temperature dependence of the linewidth of the internaldeformation mode ν₂ reveals coupling to reorientational motions of theborohydride ion in the orthorhombic phase. The thermal evolution of bothcrystal structure and vibration frequencies agree with the phase diagramsuggested by the Landau theory.

The first crystal structure of a 3d-metal borohydride is presented.Solvent-free homoleptic manganese borohydride Mn(BH₄)₂ forms at ambient conditions in ball-milled mixtures of alkali metal borohydrides and MnCl₂. It crystallizes in the trigonal crystal system with the space group symmetry P3₁12 and is stable from 90 to 450 K, where the compound melts. Thermal expansion of Mn(BH₄)₂ between 90 and 400 K is highly anisotropic and strongly nonuniform. The structure of Mn(BH₄)₂ shows interesting similarity to α-Mg(BH₄)₂: the two structures are made of similar layers L with the composition M₄(BH₄)₁₀ per cell. The layers are stacked along the c-axis, and rotated by 120° by the 3₁ axis in Mn(BH₄)₂ and by 60° by the 61 axis in α-Mg(BH₄)₂. Three identical layers are stacked along one unit cell vector c in Mn(BH₄)₂, while six layers are stacked in α-Mg(BH₄)₂. In Mn(BH₄)₂ the layers L are connected directly, and share atoms. In α-Mg(BH₄)₂ the layers L are intercalated by a thin layer L', which contains one Mg atom per layer per cell. The layer L is chiral, and both borohydrides crystallize in chiral space groups. Similar to α-Mg(BH₄)₂, the structure of Mn(BH₄)₂ is not densely packed and contains isolated voids with the estimated volume of 21 ų each, which occupy in total 6% of the space. The resemblance between Mn(BH₄)₂ and α-Mg(BH₄)₂ is also reflected in their Raman and infrared spectra.

Electronic absorption spectrum of 1 in DMF solution at room temperature, together with the calculated oscillator strengths.

We have demonstrated experimentally by Raman analysis that 1a exists as a mixture of ttt/cct conformers at 22°C in liquid or CCl₄ solution, thus contrasting with the initial IR, NMR and X-ray analyses, which were strongly in favour of (ttt)-1a. The global stereoelectronic stabilization is ca. 4.0-4.2 kcal/mol for both parallel N lone pairs (lp) in (cct)-1a, based on a ΔH of 0.75 kcal/mol (±10%), as measured by Raman spectroscopy from 22 to 90°C, as well as the roughly estimated MM2 MeNHEt and MeNHCH₂NH₂ gauche interactions. DFT calculations using B3LYP/6-31G^* yield a standard ΔH value of 1.04 kcal/mol, in good agreement with the experiment, and predict a ttt/cct ratio of ca. 77:23 at 25°C. Broadening of the ¹³C-NMR signals was observed in either CCl₄ or CS₂ or even CDCl₃ solutions between -20 and -40°C.

Geometry of [Mg(BH₄)₄] units in R-Mg(BH₄)₂, coordination of Mg₂ atom is given as an example. Two nearly planar BH₂-Mg-H₂B fragments are situated at nearly 90° dihedral angle. The shortest H · · · H distances are highlighted in green.

(a) IXS spectrum recorded at (310) (solid diamonds) compared to the resolution function (solid line) . (b) Phonon dispersion in high-symmetry directions; experimental points empty symbols connected by a guide for the eyes (solid lines) are compared with the ab initio calculations (dashed lines) for the F43m structure. The estimated experimental errors are less than the symbol size.

LiSc(BH₄)₄ has been prepared by ball milling of LiBH₄ and ScCl₃. Vibrational spectroscopy indicates the presence of discrete Sc(BH₄)₄⁻ ions. DFT calculations of this isolated complex ion confirm that it is a stable complex, and the calculated vibrational spectra agree well with the experimental ones. The four BH₄⁻ groups are oriented with a tilted plane of three hydrogen atoms directed to the central Sc ion, resulting in a global 8 + 4 coordination. The crystal structure obtained by high-resolution synchrotron powder diffraction reveals a tetragonal unit cell with a = 6.076 Å and c = 12.034 Å (space group P-42c). The local structure of the Sc(BH₄)₄⁻ complex is refined as a distorted form of the theoretical structure. The Li ions are found to be disordered along the z axis.

IR and Raman data were obtained from α-, β-, and mixed (β,γ)-Ca(BH ₄) ₂ samples and from the deuterated β,γ phase mixture. The results obtained with α phase indicate that the DFT calculated values for the B−H stretching modes and the lattice vibrations are fairly close to the experimental values. The spectral behavior at temperatures around the transition to the β phase shows a continuous transition and suggests the presence of disorder caused by reorientational motions of the [BH ₄] ⁻ ion in the β phase. The data indicate that there are more deformation bands observed for the mixed (β,γ) samples than for the α phase which indicates structural variations between the β and the γ phases.

NaBH₄·2H₂O and NaBH₄ were studied by single-crystal X-ray diffraction and vibrational spectroscopy. In NaBH₄·2H₂O, the BH₄^- anion has a nearly ideal tetrahedral geometry and is bridged with two Na⁺ ions through the tetrahedral edges. The structure does not contain classical hydrogen bonds, but reveals strong dihydrogen bonds of 1.77-1.95 Å. Crystal structures and vibrational spectra of NaBr·2H₂O and NaBH₄·2H₂O reveal many similarities. The unit cell volume of NaBH₄·2H₂O increases linearly with temperature between 200 and 313 K. At 313-315 K, the hydrate decomposes into NaBH₄ and H₂O, which react to release hydrogen.

We report a comprehensive THz, infrared and optical study of Nb-doped SrTiO₃ as well as dc conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of an unusually narrow (<2  meV) Drude peak. For all carrier concentrations the Drude spectral weight shows a factor of three mass enhancement relative to the effective mass in the local density approximation, whereas the spectral weight contained in the incoherent midinfrared response indicates that the mass enhancement is at least a factor two. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation.

A series of mixed crystals with general formula Ba_7-xNa_yF₁₂Cl_2-zBr_z in the ordered modification (space group P-6) has been studied by single crystal x-ray diffraction. Depending on synthesis conditions, the disorder in the channels (i.e. occupation of 0 0 z sites) can be changed. The disorder is found to be correlated with the refined Na content, and its effect on Ba-Cl(Br) bond length is discussed.

The luminescence of Sm²⁺ substituting for Sr²⁺ or Ba²⁺ has been studied in SrFBr, BaFBr, BaFI and SrAlF₅.

The pressure induced shifts of the intra-configurational ⁵D_0,1 → ⁷F_0,1,2 observed in the MFX crystals are about three times larger than those observed in ruby, confirming thus some potential of these systems as pressure sensors.

The comparison of excitation spectra in MFX shows that the position of the lowest 4f⁵5d¹ band shifts strongly to the red passing from SrFBr to BaFI. In BaFI, one observes simultaneously intra-configurational and inter-configurational emission.

The non-degenerate ⁵D₀ → ⁷F₀ emission of Sm²⁺ in SrAlF₅ confirms the presence of four crystallographic sites for Sr. Site selective spectra show clear differences for the different sites. Spectra as a function of pressure reveal different pressure shifts for the different sites.

The charge recombination dynamics of excited donor−acceptor complexes consisting of hexamethylbenzene (HMB), pentamethylbenzene (PMB), and isodurene (IDU) as electron donors and tetracyanoethylene (TCNE) as electron acceptor in various polar solvents has been investigated within the framework of the stochastic approach. The model accounts for the reorganization of intramolecular high-frequency vibrational modes as well as for the solvent reorganization. All electron-transfer energetic parameters have been determined from the resonance Raman data and from the analysis of the stationary charge transfer absorption band, while the electronic coupling has been obtained from the fit to the charge recombination dynamics in one solvent. It appears that nearly 100% of the initially excited donor−acceptor complexes recombine in a nonthermal (hot) stage when the nonequilibrium wave packet passes through a number of term crossings corresponding to transitions toward vibrational excited states of the electronic ground state. Once all parameters of the model have been obtained, the influence of the dynamic solvent properties (solvent effect) and of the carrier frequency of the excitation pulse (spectral effect) on the charge recombination dynamics have been explored. The main conclusions are (i) the model provides a globally satisfactory description for the IDU/TCNE complex although it noticeably overestimates the spectral effect, (ii) the solvent effect is quantitatively well described for the PMB/TCNE and HMB/TCNE complexes but the model fails to reproduce their spectral effects, and (iii) the positive spectral effect observed with the HMB/TCNE complex cannot be described within the framework of two-level models and the charge redistribution in the excited complexes should most probably be taken into account.

SrMgF₄ was prepared by precipitation in aqueous solution. Alkaline earth metal acetates and ammonium fluoride were used as precursors. After drying and annealing the samples at different temperatures and times, single phase SrMgF₄ was obtained. By varying the annealing conditions, the mean crystallite size could be adjusted. Furthermore, the thermally treated samples displayed UV-excited intensive broad band luminescence in the visible region. The emissions colour and intensity can be adjusted by the tempering conditions. X-Ray diffraction, TEM-microscopy, fluorescence and IR-spectroscopy were used for analysis.

A new barium silico-aluminate phase with the stoichiometry Ba13.35(1) Al30.7 Si5.3 O70 has been found and characterized. The compound crystallizes in the space group P63 /m (No. 176) with a = 15.1683(17) Å, c = 8.8708(6) Å, V = 1767.5(4) ų , Z = 1, Rw = 0.026, 32 refined parameters. A 3-dimensional matrix of Al/SiO4 tetrahedra with Ba(II) ions located in channels along the c axis builds up the structure. One of these channels is partially filled with Ba(II) ions (CN 6+3) in Wyckoff position 2a, leaving ∼ 1/3 of the positions empty. The second and third type of Ba(II) ions occupy channels orientated along the c axis with CN 4+2+2 and 4+3+1, respectively. The structure shows a rare clustered arrangement of six tetrahedra filled exclusively by Al(III) and therefore is an exception to Loewenstein’s rule. The other tetrahedral positions show an Al to Si ratio of ∼ 4 : 1. The Al/Si–O bond lengths in the tetrahedral Al/Si positions drawn vs. site occupation show linear behavior similar to the prediction by Vegard’s rule for solid solutions. After doping with Eu(II) the compound shows bright orange-yellow luminescence with an unusual large shift of the Eu(II) emission band.

The influence of pressure on the structural and vibrational properties of a₂RuH₆has been investigated using periodic density functional theory calculations performed at the local density approximation (LDA) and generalized gradient approximation (GGA) levels. At ambient pressure, the calculated structure and vibrational frequencies are in satisfactory agreement with experimental data. The calculated em>P-Vcurves could be fitted with the Vinet equation of state, yielding em>B₀=67.6and em>B₀=58.5  GPaat the LDA and GGA levels, respectively, and em>B₀^′=4.0at both theoretical levels. The unit cell parameter is found to decrease faster with increasing pressure than the Ru–H bond length. The calculated pressure dependence of the vibrational frequencies agrees well with experiment for em>ν₅(T_2g)but not for em>ν₉(A_1g)

Unexpected structural complexity: Well-crystallized Mg(BH₄)₂ powder is obtained, allowing the structure to be determined from synchrotron X-ray and neutron diffraction data. Mg(BH₄)₂ is a novel and remarkably complex three-dimensional framework in which each Mg²⁺ ion (blue) is tetrahedrally coordinated by four [BH₄]^- tetrahedra (B red, H gray; see picture).

Single crystal dixanthinium tetrachlorozincate has been grown from dilute chloridric acid. Polarized Raman spectrum of the single crystalline sample, FT-Raman and FT-IR spectra of the polycrystalline samples have been examined and the bands assigned to the appropriate modes predicted by a factor group analysis for the space group Pmn2₁. The crystal structure has been confirmed by powder XRD measurements.

Mixed single PbFBr_1−xI_x crystals have been prepared. X-ray powder diffraction structure determinations show that all samples crystallize with the matlockite structure. However, the single crystal structure of PbFBr_0.5I_0.5 involves not only fractional occupation of one site corresponding to the stoichiometry, but also split positions of the Pb²⁺ ion. Raman spectra reveal the presence of new additional bands with respect to PbFBr and PbFI. DFT calculations of lattice vibrations for PbFI show good agreement with experimental spectra. The calculated phonon dispersion curve suggests that for the mixed crystals the centre of inversion is conserved locally. These combined results suggest the presence of domains with ordered F–Pb–Br–Br–Pb–F and F–Pb–I–I–Pb–F layers in the mixed crystals. Calculations on PbFBr_0.5I_0.5 show that this suggested structure is more stable than the structure consisting of the F–Pb–Br–I–Pb–F arrangement.

About 3 µm thick tungsten trioxide film electrodes consisting of partly sintered, 40-80 nm in diameter, particles deposited on conducting glass substrates exhibit high photon-to-current conversion efficiencies for the photooxidation of water, exceeding 70% at 400 nm. This is facilitated by a ca. 40% film porosity resulting in high contact area with the electrolyte. It is shown that the activity of the WO₃ electrodes towards photooxidation of water is enhanced by addition of even small amounts of halide (Cl^-, Br^-) ions to the acidic electrolyte. Photoelectrolysis experiments performed either in acidic electrolytes containing chloride or bromide anions or in a 0.5 M NaCl solution, under simulated 1.5 AM solar illumination, demonstrated long term stability of the photocurrents. Oxygen remains the main product of the photoanodic reaction even in a 0.5 M NaCl solution, a composition close to the sea water, with chlorine accounting for ca. 20% of current efficiency.

The crystal structure of the disordered modification of Ba₇F₁₂Cl₂ has been carefully re-examined on several new crystals prepared under different conditions of synthesis. All single crystal structure refinements reveal a residual electron density of ~3 e^-/ų in the 0,0,0 position which is explained by the introduction of a small amount of sodium ions in the crystal. The title compound transforms from a disordered to an ordered modification at ~800 °C. New structural data show a change in space group from P6₃/m to P6. During this process, a slight chemical change and the formation of nano-channels in the crystals is observed by electron microscopy. These changes were further studied by electron microprobe analysis, by spectroscopic methods and thermal analysis.

Vibrational spectra of BH₄^- and its isotopic analogues in a crystalline environment of alkali metals cations (K⁺, Rb⁺, Cs⁺) have been investigated beyond the harmonic approximation using a variational approach supported by computations of B3LYP type anharmonic force fields. From the comparison of the observed and simulated IR spectra, the influence of the anharmonic couplings on the band position and on the relative intensity of the allowed vibrational transitions is discussed. Here, the effect of the crystalline environment induces a blue shift of about 50 and 100 cm^-1 respectively for the bending and stretching modes of BH₄^-. Furthermore, anharmonic effects, which are exclusively well reproduced by a variational approach, are needed to yield reliable positions and relative amplitudes of IR allowed combination and overtone transitions. This leads to theoretical results fitting their experimental counterpart between 6 and 30 cm^-1 in the investigated series.

Experimental (IR and Raman) and theoretical (Kohn-Sham calculations) methods are used in a combined analysis aimed at refining the available structural data concerning the molecular guests in channels formed by stacked dibenzo-18-crown-6 (DB18C6) crown ether. The calculations are performed for a simplified model comprising isolated DB18C6 unit and its complexes with either H₂O or H₃O⁺ guests, which are the simplest model ingredients of a one-dimensional diluted acid chain, to get structural and energetic data concerning the formation of the complex and to assign the characteristic spectroscopic bands. The oxygen centers in the previously reported crystallographic structure are assigned to either H₂O or protonated species.

Mixed matlokite hosts of composition BaFBr_xI_1−x(0≤x≤1) (pure and doped with Sm²⁺, Eu²⁺) were studied with x-ray crystallography, luminescence, Raman, and electron paramagnetic resonance (EPR) spectroscopy. Results are presented for BaFBr_0.5I_0.5 which demonstrate that a ferrielectric domain structure is formed due to the fact that the heavy halogen ions form separate sublattices with randomly distributed domain walls. The space group of a domain is P4  mm (No. 99). The EPR data from Eu²⁺ allowed to determine the volume fraction of domains.

The effects of milling and doping NaAlH₄ with TiCl₃, TiF₃ and Ti(OBuⁿ)₄, and of cycling doped NaAlH₄ have been investigated by infrared (IR) and Raman spectroscopy and X-ray powder diffraction. Milling and doping produce similar effects. Both decrease the crystal domain size (~900 Å for milled and ~700 Å for doped, as compared to ~1600 Å for unmilled and undoped NaAlH₄) and increase anisotropic strain (by a factor >2.5, mainly along c). They also influence structure parameters such as the axial ratio c/a, cell volume and atomic displacement amplitudes. They show IR line shifts by ~15 cm⁻¹ to higher frequencies for the Al–H asymmetric stretching mode ν₃, and by ~20 cm⁻¹ to lower frequencies for one part of the H–Al–H asymmetric bending mode ν₄, thus suggesting structural changes in the local environment of the [AlH₄]⁻ units. The broad ν₃ bands become sharpened which suggests a more homogeneous local environment of the [AlH₄]⁻ units, and there appears a new vibration at 710 cm⁻¹. The Raman data show no such effects. Cycling leads to an increase in domain size (1200–1600 Å), IR line shifts similar to doped samples (except for TiF₃: downward shift by ~10 cm⁻¹) and a general broadening of the ν₃ mode that depend on the nature of the dopants. These observations support the idea that some Ti diffusion and substitution into the alanate lattice does occur, in particular during cycling, and that this provides the mechanism through which Ti-doping enhances kinetics during re-crystallisation.

By means of ¹H-NOESY- and Raman-spectroscopic analyses, we experimentally demonstrated the presence of the equatorial N — Me conformer of King's sultam 4b in solution, resulting from a rapid equilibrium. As a consequence, the value of the N lone-pair anomeric stabilization should be revised to 1.5-1.6 kcal/mol. Independently from the N tilting, natural bond orbital (NBO)-comparative analyses suggest that the S d* orbitals do not appear as primordial and stereospecific acceptors for the N lone pair. Second, the five-membered-ring sultams do not seem to be particularly well-stabilized by the S — C σ* orbital in the N-substituted pseudo-axial conformation, as opposed to an idealized anti-periplanar situation for the six-membered-ring analogues. In this latter case, the other anti-periplanar C — C σ* and C(1') — H/C(2') σ*orbitals are as important, if not more, when compared to the S — C σ* participation. In the pseudo-equatorial conformation, γ-sultams particularly benefit from the N lone-pair hyperconjugation with the anti-periplanar S — O₁ σ* and C(2) — H/C or C(1') — H/C σ* orbitals. This is also the case for δ-sultams when the steric requirement of the N-substituent exceeds 1.6 kcal/mol. When both axial and equatorial conformations are sterically too exacting, the N-atom is prone to sp² hybridization or/and conformational changes (i.e., 12c). In that case also, the mode of stereoelectronic stabilization differs from γ- to δ-sultams.

Alkali borohydrides MBH₄ and their deuterides have been investigated by X-ray and neutron powder diffraction (M=K,Rb,Cs) and by infrared and Raman spectroscopy (M=Na,K,Rb,Cs). At room temperature the compounds crystallize with a cubic high temperature (HT) structure having Fm3m  symmetry in which the [BH₄]⁻ complexes are disordered. At low temperature (LT) the potassium compound transforms into a tetragonal low temperature structure having P4₂/n mc symmetry in which the [BH₄]⁻ complexes are ordered such as in the isotypic sodium congener. The B---H distances within the complex as measured on the deuteride at 1.5 K are 1.205(3) Å. Indications for a partial ordering in the rubidium and cesium compounds exist but are not sufficient for a full structural characterization. Infrared and Raman spectra at room temperature are fully assigned for both hydrides and deuterides, including the overtones and combination bands, the Fermi resonance type interactions and the ¹⁰B to ¹¹B splitting due to the presence of natural boron in the samples.

Temperature-dependent emission spectra of Sm²⁺-doped SrMgF₄ have been obtained in the temperature range from 50 to 300 K. At 50 K, six bands are observed for the very strong ⁵D₀→⁷F₀ transition, in agreement with the reported sixfold crystal superstructure. The overall splitting of more than 70 cm⁻¹ highlights the important structural differences of the six Sr sites. Upon heating progressively to room temperature, the spectra change progressively with a more pronounced change between 270 and 300 K. These observations suggest the possibility of a complex structural behavior for SrMgF₄ which will require new experiments.

Ba₆Mg₁₁F₃₄, a new compound of the pseudobinary BaF₂–MgF₂ system, has been synthesized by solid state techniques from stoichiometric amounts of BaF₂ and MgF₂ and its crystal structure determined by single crystal X-ray diffraction (space group P1 , a=7.5084(6), b=9.9192(8), c=10.0354(8) Å, α=81.563(2), β=72.402(2), γ=71.198(1)°, 3899 structure factors, 233 parameters, R(F²>2σ(F²))=0.018, wR(F² all) = 0.046). It is isotypic with the copper(II) analogue, Ba₆Cu₁₁F₃₄. The main features of the structure are a network of [MgF₆] octahedra and three different [BaF_x] polyhedra with x=12, 11+1 and 13. Ba₆Mg_11−xFe_xF₃₄ and Ba₆Mg_11−xMn_xF₃₄ solid solutions were prepared and their composition determined by single crystal structure analyses. The luminescence properties of Ba₆Mg₁₁F₃₄ doped with Eu²⁺ were studied using fluorescence spectroscopy. The observed luminescence was pale blue with a maximum at 465 nm.

Raman spectra of the alkali borohydride series MBH₄ (M=Li, Na, K, Rb, Cs) have been measured as a function of temperature in the range 300–540 K. For the cubic modification of M=Na, K, Rb and Cs, the analysis of the Raman line widths suggests that the energy barrier of reorientation of the [BH₄]⁻ anions decreases as a function of cation size in the sequence Na: 12.1(5), K: 9.2(4), Rb: 8.8(3) and Cs: 8.2(4) kJ/mol. For the hexagonal high temperature modification of LiBH₄, the data suggest two energy barriers of reorientation at ~5 and ~ 60 kJ/mol, respectively.

Polycrystalline LiBH₄ has been studied by Raman spectroscopy in the temperature interval 295–412 K and the frequency range 2700–130 cm⁻¹. The Raman active modes are consistent with the presence of a (BH₄)⁻ ion having a distorted tetrahedral configuration. As the temperature is increased the sudden disappearance of mode splitting points to the onset of a structural phase transition that leads to a higher local symmetry of the (BH₄)⁻ tetrahedron. The transition occurs at ~384 K, is of first-order and has a hysteresis of about 8 K. A strong and discontinuous broadening of bands remaining after the transition suggests the onset of large vibrational amplitudes of the (BH₄)⁻ tetrahedra about their trigonal axis.

Characterization of ternary and quaternary metal hydrides by Raman spectroscopy appears to be rather scarce due primarily to the decomposition of the metal hydrides by the energy of the laser excitation source. We report the results of some recent room temperature Raman measurements collected with a 2–10 mW 488 nm laser source for M₂RuH₆ where M=Ca, Sr and Eu. The assignments from this study are combined with existing vibrational data for other metal hydrides.

Crystals of ordered Ba₆EuF₁₂Cl₂ were found to form during high temperature flux growth. The structure was refined in the hexagonal space group P 6 to R^F(R^F_W) = 0.024(0.024) for 326 reflections and 46 parameters. Lattice parameters are a = b = 1059.27(8) pm and c = 416.36(2) pm; Z = 1. The structure is isotypic to Ba₇F₁₂Cl₂. No solid solution of Ba/Eu was observed, the Eu²⁺ ions are located in the channels formed by 3 + 6 fluorine ions, occupying only one of the three metal sites of the Ba₇F₁₂Cl₂ structure.

We recently discovered a new compound with composition Ba₇F₁₂Cl₂. It was possible to show that the variation of the synthesis conditions makes it possible to obtain a disordered and an ordered modification with different lattice parameters and space groups (P6₃/m [176] and P6  [174]). For Pb₇F₁₂Cl₂ an ordered modification is reported in the literature. In this paper we present the synthesis and structural characterization from X-ray diffraction data of the disordered modification of Pb₇F₁₂Cl₂. Single crystals were grown from a flux and the structure was refined in the hexagonal space group P6₃/m to R(R_w)=0.043(0.038) for 284 reflections and 26 parameters. Lattice parameters are a=b=1021.90(8) pm and c=361.93(6) pm with Z=1. Propeller-type arrangements with chlorine as axis and fluorines as blades are observed. The ordered modification of Pb₇F₁₂Cl₂ was prepared by a new hydrothermal synthesis. Differences between both modifications are found in the lattice constants and atomic occupation parameters for the atom type Pb2 and the connected fluorine ions.

Two distinct methods were used to investigate the role of Trp residues during Mg-ADP binding to cytosolic creatine kinase (CK) from rabbit muscle:  (1) Raman spectroscopy, which is very sensitive to the environment of aromatic side-chain residues, and (2) reaction-induced infrared difference spectroscopy (RIDS) and photolabile substrate (ADP[Et(PhNO₂)]), combined with site-directed mutagenesis on the four Trp residues of CK. Our Raman results indicated that the environment of Trp and of Tyr were not affected during Mg-ADP binding to CK. Analysis of RIDS of wild-type CK, inactive W227Y, and active W210,217,272Y mutants suggested that Trp227 was not involved in the stacking interactions. Results are consistent with Trp227 being essential to prevent water molecules from entering in the active site [as suggested by Gross, M., Furter-Graves, E. M., Wallimann, T., Eppenberger, H. M., and Furter, R. (1994) Protein Sci. 3, 1058−1068] and that another Trp could in addition help to steer the nucleotide in the binding site, although it is not essential for the activity of CK. Raman and infrared spectra indicated that Mg-ADP binding does not involve large secondary structure changes. Only 3−4 residues absorbing in the amide I region are directly implicated in the Mg-ADP binding (corresponding to secondary structure changes less than 1%), suggesting that movement of protein domains due to Mg-nucleotide binding do not promote large secondary structure changes.

Barium in BaMgF[4] and in Ba[6]Mg[7]F[26] can be partially replaced by Sr or Eu. The single crystal structural analysis of the title compounds (a = 409.06 pm, b = 1452.7 pm, c = 579.02 pm, space group Cmc2[1] for Ba[0.78(3)]Eu[0.22(3)]MgF[4] [Z = 2, R[w] = 0.023 for 998 reflections] and a = 583.47 pm, b = 1209.09 pm and c = 1506.56 pm, space group Immm for Ba[5.20(6)]Eu[0.80(6)]Mg[7]F[26]) MgF[4] [Z = 2, R[w] = 0.019 for 2694 reflections] confirms the substitution of barium by europium. A split refinement of the substituted barium site in both crystals reveals a trend of europium to get closer to the nearest fluoride neighbors by simultaneously reducing the coordination sphere.

Crystals of composition Ba₇F₁₂Cl₂ were obtained by a reaction at room temperature between Ba²⁺/Cl^-/F^- in a gel of agar-agar/water. The hexagonal crystals have space group P6, a=1064.69(8), c=417.89(5)pm, V=410.24(8) 106 pm3 and Z=1. The anions form a propeller type network located in tunnels parallel to the chex axis; the chloride ions are located at the center on the propeller axes. The Ba²⁺ ions are coordinated by a (distorted) tricapped trigonal environment of fluoride and chloride anions. Disorder is present for one particular Ba²⁺ site. The average structure is isotypic with the structure of Pb₇F₁₂Cl₂.

We have studied the solid-liquid equilibrium of the system Sr_1–yBa_yFCl_1–xBr_x using DTA and X-ray diffraction techniques. The entire composition range in this system yields solid solutions which crystallize in the PbFCl (Matlockite) structure type. The melting points of the entire composition range have been parametrized (within 5°C rms error) using a biquadratic fit of the available data obtained by experiment and from the literature.

Experiments in which the Raman linewidth was measured as a function of temperature (7-1183 K) and pressure (0-400 bar) were performed on the (111) and (100) planes of single crystals of the cubic anti-fluorite Li₂S. The temperature dependence of the lattice constant was determined by x-ray diffraction (11-295 K). From these results and published Brillouin scattering data for this host, the volume thermal expansion coefficient as a function of temperature was obtained as well as the isothermal compressibility and the isothermal Raman mode Grüneisen parameter. Using the thermodynamic approach within the quasi-harmonic approximation, we show that below 400 K the volume effects describe well the temperature dependence of the Raman linewidth whereas above this temperature there are direct anharmonic effects appearing. Above approximately 850 K new Raman lines appear that are A_⊥ and E polarized.

A recent investigation of the (BaF₂–MgF₂) phase diagram produced several new compounds which are suitable hosts for Rare Earth impurities. We present results on single crystals of Ba₂Mg₃F₁₀ doped with Eu²⁺. The local structure and optical properties of this system were investigated by luminescence emission and by EPR. We observed two different Eu²⁺ sites. Both show C_s point symmetry and an important ground state splitting. Correlating our EPR and optical results with the new Ba₂Mg₃F₁₀ structure data allowed the assignment of each of them to a specific barium lattice site. The luminescence emission of both the 4f⁷–4f⁶5d and the 4f⁷–4f⁷ transitions is observed. The relative importance of the two emissions is strongly temperature dependent. The emission intensities of the intra f-shell ⁶P_7/2→⁸S_7/2transitions increase strongly on going from 295 K to 77 K. Thus, the lowest levels of the 4f⁶5d configuration are approximately degenerate with the ⁶P_7/2 manifold.

Eu²⁺ was introduced into pure and oxygen codoped BaMgF₄ single crystals. A detailed EPR study of this ion (S=7/2) was realized on both types of systems. The result is that only one spectrum was observed involving a strong crystal field. The associated site symmetry of the impurity is C_s. It occupies very closely a Barium lattice site as was established by correlating the EPR results with those of a refined X-ray structure analysis on a Ba_0.8Eu_0.2MgF₄ single crystal realized in our laboratory. The oxygen codoped crystals exhibited this same Eu²⁺EPR spectrum (the only one). Optical emission and excitation experiments were performed between 13 000 and 53 000 cm⁻¹. The results due to the Eu²⁺ impurity are given and discussed qualitatively within the 4f⁷ ↔ 4f⁶5d¹ scheme.

In orthorhombic Ba6Mg7F26, the Ba2+ ion can be partially replaced by Sr2+ to form a mixed, disordered compound. This new compound has a refined composition of Ba5.24 (4)Sr0.76 (4)Mg7F26. The volume decreases after the partial substitution from 1074.64 (10) to 1064.31 (11) Å3. The structure has two barium sites, Ba1 on Wyckoff site 8(l) with Cs symmetry and a coordination number of 12 + 1, and Ba2 on site 4(j) with C2v symmetry and a coordination number of 12; only the Ba2+ on site 4(j) is partially replaced by Sr2+. The distorted octahedral fluoride environment around magnesium shows a tendency to become more irregular in the Sr-substituted compound.

Temperature-dependent Raman measurements between 190 and 358 K yield conformational enthalpy differences between 550 and 690 cal mol⁻¹ for racemic liquid 2-butanol. The conformational properties of 2-butanol were also studied using MM2 and MM3 calculations. The conformer 1, where C1 and C4 are in anti position, was found to be the most stable comformer with the MM3 calculations. Conformer 2, which has C1 and O5 in anti arrangement, has ca. 500 cal mol⁻¹ higher energy than 1. Comparisons of the calculated MM3 vibrational frequencies with the Raman spectra suggest that the most stable conformer in the liquid phase also adopts a Cl,C4 anti conformation.

The present investigation is concerned with the possible effects of material-related properties (molecular mass, glass transition and melting temperatures, crystallinity, tacticity) and particle-related properties (shape, size, specific surface area) on the compression characteristics of the chosen model polymer powder: poly(vinyl chloride) (PVC). Four grades were selected known in literature for providing compacts of varied mechanical strength. The compression characteristics were determined using an instrumented single-punch tableting machine. The differences in tableting characteristics could not be ascribed to any of the material-related properties, but a direct relationship was observed between the compact strength and the specific surface area of the particles, as measured by nitrogen adsorption. The compact hardness was thus only dependent on the inter- and the intraparticulate contact area, which in turn is dictated by the very peculiar morphology of the grains of the PVC powders, whether prepared by emulsion or suspension polymerization.

Single crystal X-ray diffraction analysis was performed on crystals with composition Ba_{1 − δ}Sr_δMgF₄ (δ ≤ 0.55). The complete structure was analyzed for single domain crystals with nominal (refined) values of δ = 0.25 (0.27(2)) and 0.5 (0.55(2)). Interatomic distances vary in a characteristic manner, when smaller strontium ions replace the barium ions. Optical studies of Sm(II) doped samples show significant inhomogeneous line broadening and confirm the disorder On the Ba Site.

A new member belonging to the binary phase diagram of BaF₂ and BaCl₂ was synthesized. The single domain crystals of Ba₁₂F₁₉Cl₅ can be prepared from a nonstoichiometric flux with molar ratio of 1 : 1 between BaFCl and BaF₂. The compound crystallizes at room temperature in the non-centrosymmetric hexagonal space group P62m with a = b = 1408.48(14) and c = 427.33(5) pm. Three different barium environements with coordination number of nine are found. The barium fluorine distances vary between 250.59(6) - a short distance compared to other Ba — F distances - and 302.7(1) pm and barium chlorine distances between 331.55(3) and 336.19(15) pm. This compound is further characterized using Raman spectroscopy.

In the structure Ba₁₂F₁₉Cl₅ [hexagonal space group P62m] the two chlorides on the sites Cl(1) and Cl(2) can partially be replaced by bromide ions. Single crystals of the type Ba₁₂F₁₉Cl_δ Br_5-δ  with a chloride to bromide ratio up to 2 : 3 could be obtained by cooling a flux of 75 mol% BaF₂ and 25 mol% BaX₂ with X = Cl, Br. The crystal quality decreases with increasing bromide concentration. Structural parameters of five selected single crystals with different chloride/bromide ratio were studied by single crystal X-ray diffraction methods. The refined total Cl^-/Br^- population ratio in the crystals is close to the one of the flux. The lattice parameters and interatomic distances change in various ways, when the smaller chloride ion is replaced by the bigger bromide ion. The refinements show a statistical disorder on the halide sites with preferential bromide substitution on site Cl(1).

Single crystals X-ray diffraction was performed on five crystals with nominal compositions Sr1/2Sm1/2FCl, Sr1/2Ca1/2FCl, Sr2/3Ba1/3FCl1/3Br2/3, Sr1/2Sm1/2FCl1/2Br1/2 and Sr0.6Sm0.1Ca 0.3FCl. Population refinements confirm the presence of correlations between lattice constants and composition. A new correlation between the heavy halogen z value and the unit cell volume is found. Luminescence spectra of bivalent samarium in these crystals contribute to the structural characterization of these compounds.

When modern spectral hole burning applications for high-density information storage under noncryogenic temperatures are envisioned, it is necessary to develop new frequency-selective photoactive materials for this purpose. Mixed compounds of the PbFCl family, doped with samarium(II) ions, exhibit promising and true room-temperature hole burning capabilities. We investigate this class of systems (and related ones) by combining material synthesis and high-resolution spectroscopy. Whole groups of isomorphous crystals were synthesized with varying degrees of halide anion and/or cation substitutions. Thin films of fluoride-based materials were made in a laboratory-built molecular beam epitaxy system. An extended x-ray study, differential thermal analysis, luminescence, and Raman measurements allowed the characterization of the materials. Formal models were developed for both the inhomogeneous zero-phonon optical line shapes of the samarium(II) and the time evolution of the hole burning.

We have developed a model to describe the inhomogeneous broadening of optical spectra in the substitutionally disordered crystals. The comparison with the experimentalf–f fluorescence spectra of SrFCl_xBr_1−x:Sm²⁺ (0≤x≤1) allowed to establish, in a very detailed manner, the relationship between the inhomogeneous spectral distribution and the crystal structure around the Sm²⁺ impurity.

The 3,6-substituted 1,2,4-trioxan-5-ones 11-14, on heating to 170-200°, underwent unimolecular thermolysis to generate electronically excited singlet ketones with an efficiency of ca. 0.2%. The chemiluminescence quantum yields (ΦoS_CL) depended on the nature of the 6-substitutents and increased linearly with temperature. The Arrhenius activation energies were obtained by measuring the rate of decay of luminescence and determined as 22.9, 30.4, 35.6, and 34.2 kcal/mol for 11-14, respectively. Step analysis of the chemiluminescence of 14 afforded an average activation energy of 44.3 kcal/mol. This latter result is explicable in terms of two decomposition paths, higher and lower in energy, leading to excited and 'dark' products, respectively. The thermolysis of trioxanones 12-14 lacking a H-atom at the 6-position is interpreted as involving successive rupture of the peroxide bond, excision of ketone at the 3-substituted end, and loss of CO₂, to finally produce ketone originating from the 6-position (see Scheme 4).

We present the results of a crystallographic and optical study realized on the system Sr_yBa_1-yFCl_xBr_1-x where x and y vary from zero to one. All mixed crystals studied were of tetragonal symmetry with the parent PbFCl structure. Complete structure determinations were performed for two single crystals with y=0.3 and x=1 respectively 0.7 (nominal composition). This system forms complete solid solutions, in contrast to previously published results.

We report on crystal growth and about physico-chemical studies on Sr_yBa_1−yFCl_xBr_1−x (y = 0, 0.5, and 1) compounds doped with Sm. Persistent spectral hole burning at 300 K is further reported on Sr_0.5Ba_0.5FCl_0.5Br_0.5:Sm single crystals.

The temperature dependent Raman spectra of the title compound confirm the presence of some rotational disorder of the NH₃ end groups below 112 K. The central carbon-carbon stretching mode around 865 cm^-1 is coupled to the order parameter of the incommensurate phase transition at 168 K. No other clear evidence of the incommensurate modulation appears in the Raman spectra between 112 and 168 K. In the Abma phase (above 168 K) a dynamic conformational gauche-trans equilibrium is observed. The corresponding enthalpy difference is estimated to be 18.4+or-6.5 kJ mol^-1.

Polarized Raman measurements on single crystals of CuO yield the symmetry assignments of the three predicted Raman active lattice modes : 297cm⁻¹ (A_g, 344cm⁻¹ (B_g) and 629cm⁻¹ (B_g). These results are compared to literature data, including IR spectra. Our measurements confirm at low temperature the appearance of an additional Raman band around 240 cm⁻¹. The temperature dependence of the linewidth of the A_g mode presents an anomalous behavior near the magnetic phase transition, suggesting the possible presence of magnon-phonon couplings in the antiferromagnetic phase.

We report on the growth of Nd_2-xCe_xCuO_4-δ single crystals (0<x<0.2) from Cu₂O flux. Free separated crystals with maximum size of 5x8x0.15 nm³ have been obtained. Magnetic AC susceptibility measurements show a sharp superconducting transition at temperatures up to 23 K. The temperature dependence of the lattice parameters has been measured by means of X-ray powder diffraction between 10 K (a=3.9413(3) Å, c=12.0290(18) Å) and 290 K (a=3.9482(3) Å, c=12.0590(18) Å). Room temperature Raman spectra reveal a new band at 320 cm^-1 which is not observed in Nd₂CuO₄. Raman spectra of crystals withT_c ranging from 7 to 22 K show a systematic intensity change of the broad band at 590 cm^-1.

The polarized Raman spectra of four different beryl crystals were studied at room temperature in the range from 30 to 4000 cm^-1. The spectra show significant differences between the samples studied, and corrections are proposed for the reference Raman spectra of beryl previously reported by Adams and Gardner (1974). Type II water is observed in two crystals; the corresponding symmetric Raman stretching band at 3595 cm^-1 is extremely strong for an impurity (about 20% of the strongest beryl lattice mode). Another, sharper, band of similar intensity at 3605 cm^-1 could possibly originate from a hydroxyl stretching mode. Additional weaker bands are observed around 1600 cm^-1 and 3600–3750 cm^-1. The first polarized Raman spectra of bazzite are presented and discussed.

Temperature-dependent Raman measurements of n-propylammonium chloride solutions in water, concentrated hydrochloric acid and methanol yield experimental gauche-trans enthalpy differences of 0.21 ± 0.13, 0.51 ± 0.26 and 0.37 ± 0.11 kcal mol⁻¹, respectively. Raman spectra in different aqueous solutions show that the gauche-trans equilibrium in solution is affected by the total chloride concentration. The gauche-trans enthalpy difference for CH₃CH₂CH₂NH₃⁺ is found by 4-31G ab initio calculations to be 0.26 kcal mol⁻¹.

Electron spin resonance of a high-T_c Bi superconductor sample is reported. The d.c. susceptibility, d.c. resistivity and a.c. susceptibility show two superconducting transitions at 105 K and 75 K. The ESR spectra show a main resonance line whose temperature dependence is studied in detail. The g value shows a maximum of 2.24 at 230 K and decreases to 2.12 at 100 K. The line width also shows a maximum of 520 G at the same temperature and drops to 200 G at 100 K. An unusual behaviour is observed in the decrease of the integrated intensity from a maximum at 230 K to below noise at level 100 K. It is possible that the origin of this signal is due to impurity phases. However, the unusual behaviour of its intensity (disappearance of the signal below T_c) may indicate that it arises from pair formation much above T_c.

We have characterized as-grown and thermally treated YBaCuO single crystals by ESR, Raman spectroscopy, magnetic susceptibility, X-ray and neutron diffraction measurements. The as-grown crystals are tetragonal and are superconducting with an onset temperature of 30 K. They show an ESR signal which behaves as a localized Cu²⁺ ion with tetragonal symmetry and presumably originates from copper chain atoms that are octahedrally coordinated by six oxygen neighbors. The temperature dependence of the ESR between 150 and 270 K shows paramagnetic behavior and also dynamical features. Below 90 K, the ESR signal disappears reversibly. A likely explanation is that the onset of local superconductivity frustrates the spins responsible for the ESR signal.

Vibrational spectroscopic methods are widely used to characterize semicrystalline polymers in terms of crystallinity. The temperature coefficient of crystallinity, an important and fundamental quantity, is seldom determined for lack of a sensitive method. In this paper, we describe an infrared approach to the measurement of the temperature coefficient of crystallinity. We start from the well-known observation that the integrated intensities of the bands in the spectrum of a semicrystalline polymer change with temperature. It is also known, though less appreciated, that only part of the change is due to changes in crystallinity, the remaining part being due to changes in the intrinsic intensity of the bands. We outline a method for separating these overlapping effects. The method has been applied to a variety of semicrystalline polyethylene samples. The temperature coefficients are found to be highly dependent both on the temperature and on the morphology of the sample. In addition we report crystallinity measurements on a solution crystallized low molecular weight (A& = 13 600) sample, discuss the origin of an apparent anomalous temperature dependence of band intensity cited in the literature, and offer quantitative evidence that the temperature dependence of specific volume is, at temperatures above 0 "C, largely determined by partial melting.

The ESR of small concentration of Gd 0.03<y<0.06 substituting for Y in single crystals of Gd_yY_1−yBa₂Cu₃O_6−x has been measured. In the insulating compound, with x ~ 0.1, and the superdconducting materials with 30 K < T_c < 80 K, the measurements were performed at X-band, 9.3 GHz, and K_α-band, 36 GHz, over a large temperature range above T_c. Angular dependence measurements exhibit a spectrum which is fully resolved in certain directions, but only partially resolved, because of exchange narrowing, in other directions. Comparisons between the spectra in the insulating and superconducting compounds shows similar angular dependent behavior. This seems to indicate that the origin of the exchange narrowing is the same in both compounds. Since this narrowing in the insulating compound arises from interaction with, or via, the Cu magnetic system, it is implied that there is a similar, perhaps fluctuating, system in the superconducting state. Preliminary measurements of the temperature dependence of the line widths may indicate the presence of spin pairing at about 110 K, above the actual T_c of 70 K. The crystal field parameters are D = 3B⁰₂ = 1307 MHz, B⁰₄ = 3.014 MHz and B⁴₄ = -11.43 MHz, for the semiconducting sample. The g-value is 1.989 ± 0.005. These values change only slightly in the superconducting crystals.

ESR results are reported on the Ag²⁺ ion introduced into alkaline earth fluoride crystals. In SrF₂ (as in CaF₂) a trigonal centre is present which tunnels between four equivalent C₃ directions as shown by ESR under uniaxial stress. A T_2g ⊗ t_2g Jahn-Teller model describes adequately the paramagnetic and stress effects when the strong coupling case is considered. Ag²⁺ in BaF₂ forms a static tetragonal cluster similar to Ag²⁺ and Cu²⁺ in SrCl₂.

The vibrational spectra of some 1,2,4-trioxanes present two characteristic bands at 790 and 880 cm⁻¹. On the basis of ¹⁸O-isotopic substitution and comparison with analogous compounds, these bands have been assigned to coupled C—O and O—O stretching modes of the C—O—O element.

A solid state example that can clearly illustrate the rigorous application of IR and Raman selection rules in a pure Td symmetry.

Glassy films of n-CzlH,, n-C36H74a, nd low molecular weight polyethylene were prepared by vacuum sublimation onto a CsI window held at 7 K and were studied by infrared spectroscopy. The conformational disorder achieved for the glass was comparable to that of the liquid near the sublimation temperature. The chain-organizing processes were monitored for Czl and polyethylene as the sample was warmed to 300 K. A number of separate steps are involved. Each step occurs over a more or less broad temperature region, and sometimes the steps overlap. In the case of CZ1, the first step involves a conformational ordering of the chains to their extended all-trans form. The extended chains initially pack in a monoclinic subcell, but they are not in longitudinal register; that is, the end methyl groups do not lie in parallel planes. At a higher temperature, the monoclinic subcell is converted to an orthorhombic subcell, but the chains are still not in register. In the last annealing step, which occurs over a narrow temperature range, the chains come into register so that the Czl finally assumes its stable orthorhombic crystal structure. The annealing of the polyethylene glass proceeds in exactly the same way except that there is no chain-registering step. In general, the transition temperatures are higher and the temperature range over which the transitions occur is broader for the polyethylene sample.

The construction of a simple high temperature Raman furnace is presented. This furnace has been operated efficiently in the temperature range from 320 to 700K.

We have obtained the room temperature Raman spectra of the three salts n-PrNH₃X (X = Cl, Br and I). The concentration of gaucheisomers increases when passing from the chloride to the iodide. We have evaluated experimentally the lattice contribution to the energy difference between gauche and anti conformers. A force field calculation yielded an energy difference of 0.85 kcal mol⁻¹ between the two conformations of the propyl-ammonium moiety.

We have obtained the room temperature Raman spectra of liquid 1,4-cyclohexadiene and its isotopic analogs: C6H7D,gem-C6H6D2, trans-C6H6D2 and C6H4D4. Additionally, we have measured the Raman spectra of 1,4-cyclohexadiene as a function of temperature from 5 to 300 K. The conformation of 1,4-cyclohexadiene is found to be very likely planar in the solid state. The solid—solid phase transition at 192 K reported by calorimetric measurements has been observed and is discussed qualitatively.

The authors have studied the Raman spectra of the crystals (EtNH3)2MCl4 (M=Cd and Mn) as functions of temperature from 5 to 300K. The external vibrations have been assigned. The space group of the Cd crystal below 114K is found to be P22/b and the phase transition P21/b-Pbca is described as a displacive first-order transition. The order-disorder transitions Pbca-Abma in (EtNH3)2MCl4 for M=Cd and M=Mn are compared. Linewidth measurements performed on the carbon-carbon stretching mode at about 870 cm-1 suggest the possibility of a second-order transition in the Mn crystal, while the first-order behaviour of the transition in the Cd crystal is confirmed.