Ultrahigh-pressure induced decomposition of silicon disulfide into silicon-sulfur compounds with high coordination numbers

Silicon disulfide, ${\mathrm{SiS}}_2$, is thought to occur in interstellar dust and is of fundamental interest more generally among the silicon chalcogenides as a comparator to ${\mathrm{SiO}}_2$, an important component of terrestrial planets. However, the high-pressure behaviors of silicon sulfides are unclear. Here, using an efficient structure search method, we systematically explore the structural evolution of different Si-S stoichiometries up to 250 GPa. ${\mathrm{SiS}}_2$ is found to be stable below 155 GPa, above which it decomposes into two compounds, SiS and ${\mathrm{SiS}}_3$. SiS adopts a high-symmetry cubic structure consisting of eightfold-coordinated silicon in face-sharing ${\mathrm{SiS}}_8$ polyhedra, while ${\mathrm{SiS}}_3$ crystallizes in a rhombohedral structure containing ninefold-coordinated ${\mathrm{SiS}}_9$ polyhedra. Analyses suggest that the Si eightfold-coordination environment could be a common feature for group IV–VI compounds under high pressure. Our findings provide insights on the nature of Si-S compounds under ultrahigh pressure.

Figure 1

Thermodynamic stability of Si-S compounds at both low and high pressures. The calculated enthalpy differences with respect to decomposition into Si and S from 0 to 250 GPa (a)–(f) are shown. Convex hulls are shown as solid lines, with stable compounds shown by solid symbols. Unstable compounds (open symbols) sit above convex hulls, with dashed lines indicating decomposition routes.

Figure 2

Calculated enthalpy difference of HP phases for the ${\mathrm{SiS}}_2$ compound with respect to the $P-3m1$ phase from 0 to 5 GPa (a) and from 260 to 420 GPa (b). (c) Calculated enthalpy differences ($\mathrm{\Delta }H$) of decomposition of ${\mathrm{SiS}}_2$ into $\mathrm{SiS}+\mathrm{S}$ and $\mathrm{SiS}+\mathrm{Si}{\mathrm{S}}_3$ relative to ${\mathrm{SiS}}_2$ as functions of pressure (50–250 GPa). (d) Schematic representation of phase diagram for stable SiS, ${\mathrm{SiS}}_2$, and ${\mathrm{SiS}}_3$ compounds as a function of pressure.

Figure 3

Phonon-dispersion curves for the $Pm-3m$ phase of SiS at 160 GPa (a) and the $R3m$ of ${\mathrm{SiS}}_3$ at 200 GPa (b). Stable structures of SiS in a $Pm-3m$ structure (c) and ${\mathrm{SiS}}_3$ in an $R3m$ structure (d).

Figure 4

Structures of the $P4/mmm$ (a) and $Cmmm$ (b) phases of ${\mathrm{SiS}}_2$. The Si atoms are eightfold coordinated by S in both structure, forming ${\mathrm{SiS}}_8$ polyhedra. Phonon-dispersion curves for the $P4/mmm$ phase of ${\mathrm{SiS}}_2$ at 300 GPa (c) and the $Cmmm$ phase of ${\mathrm{SiS}}_2$ at 450 GPa (d).

Figure 5

Comparison of the stability ranges and the silicon coordination number of ${\mathrm{SiS}}_2$ under pressure with those in its isovalent IV–VI ${AB}_2$ compounds, including ${\mathrm{CO}}_2$, ${\mathrm{CS}}_2$, ${\mathrm{SiO}}_2$, and ${\mathrm{GeO}}_2$.