Ab initiothermodynamic and elastic properties ofAGaH4hydrides (A=Li, Na, K, Rb, and Cs)

Systematic properties of the $A{\text{GaH}}_{4}$ alkali gallium hydrides ($A=\text{Li}$, 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 ${\text{LiGaH}}_{4}$, whose structure is as yet undetermined experimentally, and ${\text{CsGaH}}_{4}$. 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 ${\text{LiGaH}}_{4}$ and ${\text{NaGaH}}_{4}$ have technologically interesting standard enthalpies of formation near $\ensuremath{-}30\text{ }\text{kJ}/\text{mole}$ ${\text{H}}_{2}$. We find, however, that ${\text{LiGaH}}_{4}$ 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.