Pressure-induced phase transitions and metallization in layered SnSe

The group IV–VI monochalcogenides have attracted widespread attention because of their diverse physical properties and promising applications in electronics and optoelectronics. As a typical IV–VI semiconductor, SnSe displays ultra-low thermal conductivity and excellent thermoelectric properties, which deeply depends on its layered structure. The layered crystal structure and associated physical properties are sensitive to external pressure. Here, we have systematically investigated the structural behaviors and optical and electrical properties of layered SnSe under high pressure. The SnSe transforms from Pnma phase to Cmcm phase above 10 GPa, and a CsCl-type structure with a space group of Pm3¯m emerges around 30 GPa and coexists with Cmcm phase up to 42.5 GPa. The optical bandgap of SnSe shows gradual narrowing with increasing pressure, indicating gradual metallization of SnSe under compression. The pressure-induced metallization of SnSe is verified by electric transport experiments. The initial semiconducting SnSe transforms to a metallic state with increasing pressure up to 9.8 GPa. Both phase transitions and optical and electrical properties of SnSe at high pressure are reversible after releasing pressure. Our study provides a modulation strategy of crystal structures and physical properties for the group IV–VI monochalcogenides to broaden their applications in thermoelectric and optoelectronic fields.