Phase boundaries and molar volumes of high-temperature and high-pressure phase V of LiBH4

Raman measurements up to 14 GPa and 570 K and powder x-ray diffraction measurement from 4 to 28 GPa at 510 K were used to investigate the high-temperature and high-pressure phases of LiBH4. The B-H stretching Raman bands in high-temperature and high-pressure phase V were observed as one broad peak, which would arise from the disordered structure. The rotation of BH4− ions has also been observed by a quantum MD calculation. The results are consistent with the reported structure of phase V that is known to be an orientationally disordered structure. The Raman measurements indicated that the phase boundary between phases I and V and that between phases III and V have a negative slope against pressure. The negative slope could be explained by the Clausius-Clapeyron equation, assuming that the entropy of phase V is larger than that of phase I or III for the disordered structure of phase V. The molar volume of phase V, despite being a high-temperature phase, is smaller than that of phase III at the same pressure from powder x-ray diffraction measurement; therefore, the thermal expansion would be small compared to the volume change at the phase transition from phase III to V.