A combination of synchrotron X-ray diffraction techniques have been applied to resolve ambiguities between experimental and theoretical studies of LiBH 4 and to reveal its complex structural evolution as a function of temperature. Crystal structures of the low- and high-temperature polymorphs of LiBH 4 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 P 6 3 mc . Experimental data suggest a nearly isotropic disorder of the rigid tetrahedral BH 4 groups as one of the factors stabilizing the hexagonal structure. Tetrahedral BH 4 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 LiBH 4 published so far.
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Filinchuk et al. (2008) studied this question.