Insight into Mg(BH4)2 with Synchrotron X-ray Diffraction: Structure Revision, Crystal Chemistry, and Anomalous Thermal Expansion

Pure Mg(BH4)(2) has been characterized by single-crystal and powder synchrotron X-ray diffraction and by vibrational spectroscopies. The earlier reported P6(1) structure of the alpha-phase is revised in the space group P6(1)22. Location of the H-atoms from powder data in the published P6(1) models posed the main problem for the identification of the correct symmetry: wrongly determined orientations of some BH4 groups hampered a successful detection of the true P6(1)22 symmetry. Four nearly ideally tetrahedral BH4 groups form a dodecahedral MgH8 coordination around each Mg atom, which can be described as a slightly distorted snub disphenoid, where two nearly planar BH2-Mg-H2B fragments are situated at similar to 90 degrees dihedral angle. The H center dot center dot center dot H distances between two BH4 groups within the BH2-Mg-H2B fragments are among the shortest (2.18-2.28 angstrom) in the structurally characterized metal borohydrides. alpha-Mg(BH4)(2) contains an unoccupied void, accounting for 6.4% of space in the structure. It is large enough (37 angstrom(3)) to accommodate a small molecule, such as H2O. The high-temperature beta-phase is less dense by similar to 3% but contains no unoccupied voids. The alpha-phase transforms into the beta-phase above 490 K; the latter is quenched (metastable) on cooling. The anomalous cell expansion of the beta-phase down to 100 K may be related to the evolution of the free energy profile far from the phase transition temperature.