show an order of magnitude less variation than the P -and generally in the opposite direction, being largest for the four Pnma phases and smallest for the three Fm 3m phases. Noting again that the majority of the heat transport occurs through the lowfrequency modes (c.f. Fig. ), the Ñ2 (and also the τ CRTA and P) will be weighted towards these modes, and the trends among compounds with the same structures reflect the impact of the range of the frequency spectra on the N2 (c.f. Fig. ).