For each alloy, we obtain two predictions of 𝜅 ave as a function of composition (one for each endpoint), which we take as upper and lower bounds for predictions. Although the predictions in the previous section indicate that Bi2(Se0.55S0. 45) would adopt the Pnma structure, the steep decrease in 𝜅 latt with S content for 0 < 𝑥 < 0.1 suggests that alloying with a small amount of S may be beneficial to reducing the 𝜅 latt of R3 ̅ m Bi2Se3.