High-Pressure Polymorphism as a Step towards Destabilization of LiBH4 (original) (raw)

2008, Angewandte Chemie International Edition

Lithium borohydride could be an extremely efficient energy storage system containing 18.5 wt % hydrogen. However, owing to its high thermal stability, it is not yet regarded as a practical H-storage material. More experimental and theoretical efforts are required to improve the hydrogen-storage properties of this compound. Experimental investigations of light metal borohydrides such as LiBH 4 are difficult owing to the weak diffracting power of the light elements for X-ray diffraction and to considerable incoherent scattering by H and high absorption by natural B and Li for neutron diffraction. For these reasons, LiBH 4 has been extensively studied theoretically by "first-principles" methods based on density functional theory (DFT). A large amount of information has been generated, including possible crystal and electronic structures, lattice dynamics, surface properties, decomposition mechanisms, and intermediate products. Surprisingly, theory and experiment agreed only on the symmetry of the room-temperature, ambient-pressure polymorph of lithium borohydride. Despite the fact that the temperature-induced structural transition in LiBH 4 has been known for a long time, the experimental structural data on the high-temperature form have not yet been confirmed by theory. In particular, the presumed hexagonal P6 3 mc structure, first suggested from diffraction experiments, was found to have a relatively high energy and imaginary vibrational frequencies. Other calculations have also shown that the P6 3 mc structure is rather unstable. The same problem holds for the pressure evolution of LiBH 4 ; a phase transition below 5 GPa was identified more than 30 years ago, but there is still no agreement on the structure of the high-pressure phase. Theoretical predictions suggest a cubic NaBH 4 -type structure above 6.2 GPa, a monoclinic P2 1 /c structure at approximately 1 GPa, and a monoclinic Cc structure above 2.2 GPa (3 GPa in reference [4]). However, the most recent experimental study of LiBH 4 at pressures up to 9 GPa concludes that none of these predictions are correct, although the structure of the highpressure polymorph itself could not be identified, owing to experimental limitations. Thus, the first efforts in understanding the material properties, both experimental and theoretical, were discouraging. Both the pressure and temperature evolution of the corresponding structure have found no consistent explanation in the framework of "first-principles" theories.