Ab initio thermodynamic and elastic properties of AGaH4 hydrides (A=Li, Na, K, Rb, and Cs)

Systematic properties of the AGaH(4) alkali gallium hydrides (A=Li, Na, K, Rb, and Cs) are investigated within density functional theory. Seven ground-state crystal structures are identified, with two energetically indistinguishable structures found for both LiGaH4, whose structure is as yet undetermined experimentally, and CsGaH4. Born effective charge tensors, static and high-frequency dielectric tensors, and phonon dispersion relations incorporating longitudinal-optical/transverse-optical mode splittings are computed. Our results indicate that LiGaH4 and NaGaH4 have technologically interesting standard enthalpies of formation near -30 kJ/mole H-2. We find, however, that LiGaH4 is thermodynamically unstable with respect to both LiGa and LiH, providing a possible explanation for its challenging synthesis. The Born stability criteria are evaluated with the computed elasticity tensor components, C-ij. All seven structures are found to be both elastically and vibrationally stable.