and R3 ̅ m phases of Bi2S2, Bi2Se3 and Bi2Te3 discussed above, we would expect Bi2(Se1-xSx)3 to potentially form in the Pnma or R3 ̅ m structures, whereas we would only expect Bi2(Se1-xTex)3 to form in the R3 ̅ m structure. This is largely borne out in experimental reports, although Bi2Te2S has been reported in the R3 ̅ m structure. We therefore investigated the mixing thermodynamics of these three alloy compositions following the approach adopted in our previous work on the Snn(S1-xSex)m system. Systematically substituting the chalcogen atoms in the 20-atom Pnma Bi2S3 primitive cell yields 1,072 unique structures in 13 compositions between the Bi2S3 and Bi2Se3 endpoints, from which we analyzed every second composition (560 structures). Applying the same procedure to a 2 × 2 × 1 supercell expansion of the R3 ̅ m primitive cell yields 378 unique structures in 13 compositions between the Bi2S3/Bi2Se3 and Bi2Se3/Bi2Te3 endpoints (i.e. 756 structures in total), from which we analyzed all 13 compositions.