Pressure- and temperature-induced phase transition in the semiconductor SnS

Semiconductor SnS with excellent physical properties has attracted wide attention due to its nontoxicity and abundance in the earth. SnS crystallizes in an orthorhombic structure (Pnma) at ambient conditions. Its unit cell comprises double layers stacked with weak van der Waals--like coupling, rendering it sensitive to high temperature ($T$) and high pressure ($P$) conditions. Here we experimentally demonstrate that SnS undergoes two phase transitions under high pressure by in situ x-ray diffraction and Raman spectroscopy. The Pnma SnS transforms into the Cmcm phase above 22.4 GPa, and a CsCl-type structure ($\mathit{Pm}\overline{3}m$) emerges at around 43.2 GPa, coexisting with the Cmcm phase up to about 66 GPa. SnS completely transforms into the $\mathit{Pm}\overline{3}m$ phase by 67.9 GPa. Theoretical calculations are performed to elucidate the structural characteristics of the high pressure Cmcm phase. Optical absorption and electrical transport experiments confirm that high pressure can reduce the band gap of SnS, leading to metallization. Additionally, the $P\ensuremath{-}T$ phase diagram of SnS has been investigated by in situ Raman spectroscopy. The Pnma to Cmcm phase transition boundary of SnS at high $P\ensuremath{-}T$ conditions is depicted experimentally. The effects of high temperature and high pressure on crystal structure, and optical and electrical properties of SnS are reversible upon returning to ambient conditions. Our research offers insights into the structures and physical properties of the group IV-VI monochalcogenides under extreme conditions and their potential applications in electronics and optoelectronics.