Based on our results, alloying Bi2Se3 with a small amount of Bi2S3 would constitute a reliable route to stabilizing the Pnma phase, whereas a low-temperature synthesis route may allow an R3 ̅ m alloy with up to ~50 % S content to be formed. The former is notable as the Pnma phase is predicted to have a significantly lower thermal conductivity than the R3 ̅ m phase, which would be beneficial for the thermoelectric properties. On the other hand, the latter is potentially also interesting as R3 ̅ m Bi2Se3 and Bi2Te3 are both topological insulators. 17 We also note that while alloys with low S content may be unstable with respect to phase separation into Pnma Bi2S3 and R3 ̅ m Bi2Se3 at lower temperature, the very different structures mean this would likely require significant atomic rearrangement, and alloys formed at high temperature and quenched may therefore remain kinetically stable.