  SnxSey crystalline compounds consisting of Sn and Se atoms of varying
composition are systematically investigated at pressures from 0 to 100 GPa
using the first-principles evolutionary crystal structure search method based
on density functional theory (DFT). 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, SnSe-Cmcm phase is transformed to
CsCl-type SnSe-Pm3m phase, which remains stable at even higher pressures. A
metallic compound with different stoichiometry, Sn3Se4-I43d, is found to be
thermodynamically stable from 18 GPa to 70 GPa. Known semiconductor tin
diselenide SnSe2-P3m1 phase is found to be thermodynamically stable from
ambient pressure up to 18 GPa. Initially being semiconducting, it experiences
metalization at pressures above 8 GPa.
