High-pressure polymorphism of $\mathrm{As}_2\mathrm{S}_3$ and new $\mathrm{As}\mathrm{S}_2$ modification with layered structure

At normal pressure, the $\mathrm{As}_2\mathrm{S}_3$ compound is the most stable equilibrium modification with unique layered structure. The possibility of high-pressure polymorphism of this substance remains questionable. Our research showed that the $\mathrm{As}_2\mathrm{S}_3$ substance was metastable under pressures $P> 6$ GPa decomposing into two high-pressure phases: $\mathrm{As}_2\mathrm{S}_3\to\mathrm{AsS}_2+\mathrm{AsS}$. New $\mathrm{As}\mathrm{S}_2$ phase can be conserved in the single crystalline form in metastable state at room pressure up to its melting temperature ($470$ K). This modification has the layered structure with $\mathrm{P12}_11$ monoclinic symmetry group; the unit-cell values are $a = 7.916(2)$ Å, $b = 9.937(2)$ Å, $c = 7.118(1)$ Å, $\beta = 106.41^\circ$ ($Z=8$, density $3.44$ g/cm$^3$). Along with the recently studied AsS high-pressure modification, the new $\mathrm{As}\mathrm{S}_2$ phase suggests that high pressure polymorphism is a very powerful tool to create new layered-structure phases with “wrong” stoichiometry.