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.

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.

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.

(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.

The compound of stoichiometry Mn(II)₃[Mn(III)(CN)₆]₂·zH₂O (z = 12−16) (1) forms air-stable, transparent red crystals. Low-temperature single crystal optical spectroscopy and single crystal X-ray diffraction provide compelling evidence for N-bonded high-spin manganese(II), and C-bonded low-spin manganese(III) ions arranged in a disordered, face-centered cubic lattice analogous to that of Prussian Blue. X-ray and neutron diffraction show structured diffuse scattering indicative of partially correlated (rather than random) substitutions of [Mn(III)(CN)₆] ions by (H₂O)₆ clusters. Magnetic susceptibility measurements and elastic neutron scattering experiments indicate a ferrimagnetic structure below the critical temperature T_c = 35.5 K.