Tin-Selenium Compounds at Ambient and High Pressures

SnxSey crystalline compounds consisting of Sn and Se atoms of varying compositions are systematically investigated at pressures from 0 to 100 GPa using the first-principles evolutionary crystal structure search 0.8 method based on density functional theory. All known experimental phases of SnSe and SnSe2 are found without any prior input. A second-order polymorphic phase transition from SnSe-Pnma phase to SnSe-Cmcm phase is predicted at 2.5 GPa. Initially being semiconducting, this phase becomes metallic at 7.3 GPa. Upon further increase of pressure up to 36.6 GPa, SnSeCmcm phase is transformed to CsCl-type SnSe-Pm (3) over barm phase, which remains stable at even higher pressures. A metallic compound with different stoichiometry, Sn3Se4-I (4) over bar 3d, is found to be thermodynamically stable from 18 to 70 GPa. Known semiconductor tin diselenide SnSe2 -P (3) over bar m1 phase is found to be thermodynamically stable from ambient pressure up to 18 GPa. Initially being semiconducting, it experiences metallization at pressures above 8 GPa.