Ca (Al H4)2, CaAl H5, and Ca H2 +6LiB H4: Calculated dehydrogenation enthalpy, including zero point energy, and the structure of the phonon spectra

The dehydrogenation enthalpies of Ca (Al H4)2, CaAl H5, and Ca H2 +6LiB H4 have been calculated using density functional theory calculations at the generalized gradient approximation level. Harmonic phonon zero point energy (ZPE) corrections have been included using Parlinski's direct method. The dehydrogenation of Ca (Al H4)2 is exothermic, indicating a metastable hydride. Calculations for CaAl H5 including ZPE effects indicate that it is not stable enough for a hydrogen storage system operating near ambient conditions. The destabilized combination of LiB H4 with Ca H2 is a promising system after ZPE-corrected enthalpy calculations. The calculations confirm that including ZPE effects in the harmonic approximation for the dehydrogenation of Ca (Al H4)2, CaAl H5, and Ca H2 +6LiB H4 has a significant effect on the calculated reaction enthalpy. The contribution of ZPE to the dehydrogenation enthalpies of Ca (Al H4)2 and CaAl H5 calculated by the direct method phonon analysis was compared to that calculated by the frozen-phonon method. The crystal structure of CaAl H5 is presented in the more useful standard setting of P 21 c symmetry and the phonon density of states of CaAl H5, significantly different to other common complex metal hydrides, is rationalized.