where L is the Lorentz number. In these systems, the decrease in σ counteracts the increase in T to produce a relatively flat temperature dependence of the κ el (Fig. ). On the other hand, the κ el of the R3m phases are predicted to rise sharply above around 500 -600 K, from < 1 W m -1 K -1 to ∼ 3.5 -4.5 W m -1 K -1 at T = 900 K. The σ of the R3m phases is predicted to fall at low temperature, reach a minimum at intermediate T , and rise at higher T , and the latter would combine with the T term in Eq. 2 to produce this behaviour.