The normalised P in Table span two orders of magnitude and show a clear variation with structure type. The four Pnma phases have the smallest values, while Fm 3m GeTe and GeSe have the largest. Fm 3m SnTe appears to be an outlier with an order of magnitude smaller P than the other two Fm 3m structures, which accounts for the longer τ CRTA highlighted in the preceding discussion. The R3m GeTe and SnTe phases and Cmcm SnSe all have intermediate values.