Lightest Borohydride Probed by Synchrotron X-ray Diffraction: Experiment Calls for a New Theoretical Revision

A combination of synchrotron X-ray diffraction techniques have been applied to resolve ambiguities between experimental and theoretical studies of LiBH4 and to reveal its complex structural evolution as a function of temperature. Crystal structures of the low- and high-temperature polymorphs of LiBH4 have been determined from diffraction on single-crystals. In contrast to recent theoretical conjectures, we find that the high-temperature structure is hexagonal, space group P63mc. Experimental data suggest a nearly isotropic disorder of the rigid tetrahedral BH4 groups as one of the factors stabilizing the hexagonal structure. Tetrahedral BH4 anions are undistorted and geometrically very similar in both polymorphs. The first order phase transition at 381 K is preceded by highly anisotropic lattice expansion and is accompanied by a negative volume change. Disorder phenomena and strong lattice anharmonicity, being ignored, lead to the failure of theoretical predictions of the structural stability of LiBH4 published so far.