Theoretical and experimental investigation of the equations of state and phase stabilities of MgS and CaS

The equations of state and phase stabilities of MgS and CaS are investigated via non-empirical theoretical calculations using three different electron-gas models: the self-consistent ion breathing (SCIB), the variationally induced breathing (VIB) and the potential-induced breathing (PIB) models. We apply these models on an equal footing using Kohn-Sham ionic densities and identical interaction density functionals. The calculated equations of state are compared to the compression curves of MgS at pressures up to 54 GPa and of CaS at pressures up to 52 GPa. The accuracies of the three electron-gas models in reproducing the equations of state of both compounds are generally comparable to those previously achieved for the binary oxide and halide systems. We also investigate the phase stabilities of MgS and CaS in CsCl (B2), wurtzite (B4) and zincblende (B3) structures. Our calculations accurately determine the B1-B2 phase transition for CaS. In the case of MgS, the transition pressure is much higher than that of the current experimental measurement ranges. In addition, the models predict that the B4 phases of MgS and CaS can be stabilized under moderate tensions. This result is consistent with experimental observation of epitaxially stabilized MgS wurtzite films.