As noted above, with a 𝑇 F of 800 K we find that the Bi2SeTe2 composition is heavily skewed towards a small number of structures, which makes it feasible to compute an approximate 𝜅 latt from explicit calculations on these structures. We select the two most probable structures, with 𝑃 𝑛 = 0.42 and 0.2, and compute an averaged 𝜅 latt as a function of temperature from a partial weighted sum according to (c.f. Eq. 12): where the sum runs over a subset 𝑛 𝑥 ′ = 2 of the 𝑛 𝑥 = 21 structures in the model and we have omitted the dependence on the composition 𝑥 for brevity . Figure shows the predicted temperature dependence of the 𝜅 latt of Bi2SeTe3 obtained for 𝑇 F = 800 K. We also compare this to two models using the Bi2Te3 force constants, viz. one where we change the mass of the Te atom at the interior Chal site from 𝑚 Te to 𝑚 Se , and a second where we use an average mass and isotope-scattering model to approximate a random distribution of Se and Te over both chalcogen positions. The latter third model is equivalent to the analysis in Figure . Surprisingly, and in contrast to experiments, we find that the partial weighted sum predicts a 𝜅 latt higher than Bi2Te3. The most probable of the two structures with 𝑃 𝑛 = 0.42 corresponds to an ordered phase where the Chal and Chal sites are fully occupied by Se and Te atoms. The calculated in-plane and out-of-plane 𝜅 latt of this configuration at 𝑇 = 300 K is 2.55 and 1.37 W m - 1 K -1 , compared to 1.01/0.61 and 2.26/0.94 W m -1 K -1 in Bi2Te3 and Bi2Se3 respectively. As discussed previously, Se would likely show stronger ionic bonding in the Chal site and Te would show stronger vdW interactions in the Chal position, so this structural arrangement of Bi2SeTe2 may in fact optimize the chemical bonding strength compared to either endpoint.