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 Mg0.5Mn0.5(BH4)2. MgxMn(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 MgxMn(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 MgxMn(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 Mg0.5Mn0.5(BH4)2 samples. An eutectic relation between α-Mg(BH4)2 and LiBH4 is observed. The solid solution MgxMn(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.
 
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.
  • Correction: The First Crystallographic and Spectroscopic Characterization of a 3d -Metal Borohydride: Mn(BH4)2
    R. Cerny, N. Penin, H. Hagemann and Y. Filinchuk
    Journal of Physical Chemistry C, 113 (32) (2009), p14582-14582
    DOI:10.1021/jp9062687 | Abstract | Article HTML | Article PDF
Pages 9003−9007. The author improved the information in the CIF file in Supporting Information. The manuscript was published on the Web on April 9, 2009 (ASAP) and in print (Volume 113, Issue 20). The correct version was published on the Web on July 14, 2009.
  
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 Å3 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.
  • LiSc(BH4)4: A Novel Salt of Li+ and Discrete Sc(BH4)4− Complex Anions
    H. Hagemann, M. Longhini, J.W. Kaminski, T.A. Wesolowski, R. Cerny, N. Penin, M.H. Sørby, B.C. Hauback, G. Severa and C.M. Jensen
    Journal of Physical Chemistry A, 112 (33) (2008), p7551-7555
    DOI:10.1021/jp803201q | unige:3567 | Abstract | Article HTML | Article PDF
  
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.

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