where L is the Lorentz number, which in general is not constant. We therefore predict a negligible κ el of <0.1 W m -1 K -1 for the π phases with n = 4 × 10 19 compared to ∼1 W m -1 K -1 for ntype Pnma SnSe under the same conditions. This analysis shows that optimising the electrical properties of the π phases requires n-type doping at a level comparable to, or ideally higher than, the p-type doping levels reported in experiments on Pnma SnSe. The electrical conductivity is given by: