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

Abstract

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.

Phase transformation of BeS and equation-of-state studies to 96 GPa

“…1. The dependence on the ionic radius ratio was first shown in part by Syassen 4 and expanded by Ekbundit et al 5 The references for the individual data points are given in the legend of Fig. 1.…”

“…23 The corrected equation of state ͑EOS͒ is expected to be somewhat softer. 5 The measurements of the first experiments suffice to obtain the equation of state and the structure of the new phase. However, it was decided to make a second study with no marker material since the sample itself could now serve as the marker.…”

“…1. [3][4][5][6][7][8][9][10][11][12][13][14] ͑In the case of BaO, the pressure at which it transformed to a distorted CsCl structure was used.͒ Figure 1 shows the expected transition pressures to eightfold coordination for the MgX chalcogenides and for the BeX chalcogenides based on this radius ratio criteria. The asterisk shows the Goldschmidt radius ratio of 0.155 that is the dividing point between two-dimensional structures and threedimensional structures.…”

The Crystal Structures of Mg3N2and Zn3N2

“…In that case a recent reexamination (14) showed that the originally assigned unit cell and space group were in fact incorrect. In view of the fact that it has recently been shown (15) that properties such as the equation of state of simple sulfides such as MgS and CaS can be successfully modeled using an ionic model, accurate structural data for simple nitrides should be of interest in that connection also.…”

Metastable group II sulphides grown by MBE: surface morphology and crystal structure

“…Both materials are under slight tensile strain (Table 1). Ekbundit et al [33] have calculated the stabilities of different MgS structures. They calculated that the ZB and wurtzite phases can be stabilised under a negative hydrostatic stress (negative pressure) of 6.9 GPa for wurtzite, and higher for NaCl.…”