Barring two exceptions, the materials can be grouped by structure type. Cmcm SnSe has the lowest κ iso , followed by Pnma SnTe, GeTe and SnSe, then Fm 3m GeSe and GeTe, and finally R3m SnTe and GeTe. Cmcm SnSe is therefore predicted to have the smallest κ iso across all ten systems, which is consistent with its exceptional thermoelectric performance. The two exceptions to the trend are Pnma GeSe, which falls between the two lighter Fm 3m systems and the R3m systems, and Fm 3m SnTe. Among the four Pnma systems, the ordering in the 300 K thermal conductivity of SnTe < SnSe ≃ GeSe < GeSe largely follows the average masses in Table . We therefore do not consider the κ iso of GeSe to be an outlier. The same holds for the two R3m phases, i.e. the κ iso of SnTe is lower than that of GeTe, but with just two data points we cannot really consider this a trend. On the other hand, Table Analysis of the lattice thermal conductivity of the ten compounds examined in this work at T = 300 K. For each compound we show the three diagonal components of the κ κ κ tensor, κ xx , κ yy and κ zz , and the isotropic average κ iso defined in Eq. 7, together with the harmonic function (κ κ κ/τ CRTA ) iso and averaged lifetime τ CRTA defined in Eq. 9. We also show the integrals of the N2 defined in Eq. 13 from 0 to the maximum frequencies f max in the phonon spectra, and the averaged phonon-phonon interaction strength P and number of scattering channels Ñ2 defined in Eqs 12-17. while Fm 3m GeSe has a lower κ iso than GeTe, as expected, SnTe, with the largest average mass, has a much larger predicted κ iso m and therefore does represent an outlier here.