At a fixed n = 10 21 cm -3 the σ decrease with temperature in all three systems, indicative of degenerate semiconductors with metallic-like conductivity. On the other hand, the absolute values of the the S increases with temperature from 100 µV K -1 at 200 K to around 200-250 µV K -1 at 1200 K. This balance results in a rise in the PF from 300 K and a subsequent levelling off from ∼ 750 K. The temperature dependence of the κ el mirrors the behaviour of the electrical conductivity, in keeping with the Wiedemann-Franz law, but at this modest n are between 0.1-0.6 W m -1 K -1 .