We computed the speed of sound (v s ) of each alloy composition from the bulk modulus (B) using the approximate relation v s = p B/r, where r is the density. Figure shows the variation of v s and B, both of which linearly increases with the alloy composition x following Vegard's law. Replacing Ba in the parent compound, Ba 2 CdP 2 , with Sr, which is less electropositive and has smaller atomic mass, makes the alloy stiffer (higher B). However, due to the small change in B (< 2 GPa) and v s (< 0.15 km/s) between x = 0.35 0.70, we do not expect a significant variation in k L in this range due to lattice softening/stiffening. As a result, after considering electronic transport and k L , the optimal composition range is still x = 0.35 0.70. Therefore, we choose one of the compositions within this range, Ba 0.75 Sr 1.25 CdP 2 (x = 0.625) to check if this optimal compositions is n-type dopable.