The second model for the 𝜅 latt of Bi2SeTe2, using the Bi2Te3 structure and force constants but with the mass of the Chal ion adjusted from 𝑚 Te to 𝑚 Se , gives remarkably similar results to the partial weighted sum. This model incorporates the increased Chal mass but neglects the enhanced bond strength, and thus gives a result intermediate between the two explicit calculations in the partial weighted sum. Finally, the third model, assuming a random distribution of Se and Te over the three chalcogen sites, predicts a 300 K 𝜅 ave of 0.59 W m -1 K -1 , which is a ~30 % reduction compared to Bi2Te3. This larger than the 10 % observed experimentally, and emphasizes the impact of the inhomogeneous distribution of chalcogen atoms predicted in the previous section. shows remarkable agreement with experiment, and suggests that the stability window over which the R3 ̅ m alloy forms could possibly be extended using low-temperature synthesis routes.