Chemical and Lattice Stability of the Bismuth Chalcogenides. we identified four reported crystal phases across the three chalcogenides Bi2S3, Bi2Se3 and Bi2Te3 (Figure ). Bi2S3 is reported in an orthorhombic Pnma phase with 20 atoms in the primitive cell. Bi2Se3 is reported in an equivalent Pnma phase and rhombohedral R3 ̅ m and tetragonal P42/nmc phases with five and 40 atoms in the respective primitive cells. Finally, two R3 ̅ m phases of Bi2Te3 are reported, both with five atoms in the primitive cell; the first of these is isostructural with the R3 ̅ m phase of Bi2Se3, while the second, which we denote R3 ̅ m*, has different ordering of atoms within the quintuple layers along the crystallographic c axis. S, Se and Te atoms are colored yellow, green and olive, respectively. These images were prepared using the VESTA software. Since the Pnma and R3 ̅ m phases are common to more than one chalcogenide, we also performed calculations on Bi2Te3 in the Pnma structure and Bi2S3 in the R3 ̅ m structure. In total, we therefore investigated eight structures: two of Bi2S3, and three each of Bi2Se3 and Bi2Te3.