The ZT max of the Pnma phases are all achieved at higher doping levels and temperatures. Higher n maximise the electrical conductivity, which dominates the power factor S 2 σ , while the shallow temperature dependence of the power factor S 2 σ and electronic thermal conductivity κ el mean that the reduction in κ latt favours higher T (cf. Figs. 4/6(a)). In contrast, the ZT max of the R3m phases occur at the largest n = 10 20 cm -3 we tested but at lower T . In these systems, larger n maximises the S, σ and PF and reduces the κ el , whereas the mid-temperature maximum in the PF and the opposing increase and decrease in the κ el and κ latt , respectively, at higher temperatures produce a ZT max between 650 -750 K.