The narrow bandgap of the R3m phases results in large intrinsic p-type conductivity which dominates the electrical properties up to n ≈ 10 17 cm -3 . Above this, both the σ and S increase with larger doping levels, and the increase in the electrical conductivity results in a steep rise in the PF above n ≈ 10 19 cm -3 . Similar behaviour was also predicted for rocksalt SnS and SnSe in our previous work. The S of the R3m phases is smaller than the Pnma phases at low n but comparable at large n. The σ obtained with p-type doping is larger than in the p-doped Pnma phases over the range of n tested, resulting in a 3 -4× larger PF at n = 10 20 cm -3 . n-type doping produces a smaller σ , comparable to the ndoped Pnma systems, and the balance of the smaller S and steeper dependence of the σ on n results in lower PFs than predicted for the n-doped Pnma systems at most doping levels.