Fig. 4. Formation enthalpy and phonon calculations.

a Formation enthalpy ΔH in the reaction MH + M₂Ch → M₃HCh (M = Li, Na) under 0 and 5 GPa. b Phonon band structures and partial phonon density of states (DOS) of Li, H, and S atoms in Li₃HS under 0 GPa and c 5 GPa. The negative number of the vertical axis represents imaginary frequency. d HM₆ octahedral rotation associated with the phonon mode denoted by yellow circles in phonon band dispersion, where ω_rot is the frequency of the HM₆ rotational mode at M point. e Displacement ellipsoids of Li atoms depicted at 50% probability level obtained from Rietveld refinement of ND in Li₃HS.