Tin–Selenium Compounds at Ambient and High Pressures

Sn_<i>x</i>Se_<i>y</i> 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 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-<i>Pnma</i> phase to SnSe-<i>Cmcm</i> 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, SnSe-<i>Cmcm</i> phase is transformed to CsCl-type SnSe-<i>Pm</i>3̅<i>m</i> phase, which remains stable at even higher pressures. A metallic compound with different stoichiometry, Sn₃Se₄-<i>I</i>4̅3<i>d</i>, is found to be thermodynamically stable from 18 to 70 GPa. Known semiconductor tin diselenide SnSe₂-<i>P</i>3̅<i>m</i>1 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