Theoretical Study of Alkali-Metal Hydrides at High Pressures: A Case of NaH Supported by Inelastic Neutron Scattering (INS) Experiments at 1 and 2 GPa.

Pressure-induced effects in alkali hydrides are investigated using a plane-wave density functional theory method. For the first time, we have measured the inelastic neutrons scattering (INS) spectra of NaH at pressures 1 and 2 GPa and used it to validate INS simulated from the first-principles calculations using both local density approximation (LDA) and the generalized gradient approximation (GGA). We found that LDA describes lattice dynamics better compared to the GGA. Thermodynamic properties such as lattice parameters, bulk modulus, and their derivatives are calculated using full lattice dynamics theory within the quasi-harmonic approximation (QHA) for all alkali hydrides. Anharmonic effects are investigated for NaH from the molecular dynamics trajectories and are negligible at given temperature and pressures. We have shown that the phase-change pressures obtained from the equal Gibbs free-energy conditions for two phases compare well with the available experimental data and is the accurate phase-change criterion. This study corroborates INS as an important complementary tool in benchmarking first-principles calculations.