Table summarizes the Ba and Sr occupancy of sites 1 and 2, the cell parameters (a, b, c), and unit cell volume (V ) as a function of the alloy composition x. Our predicted site selectivity is confirmed by experiments. For Ba-rich alloys (x < 0.5), site 1 is The free energy of mixing (DG m ) of Ba 2(1 x) Sr 2x CdP 2 alloys as a function of x calculated at 900 K, the typical synthesis temperature of these Zintl phases. Labels (A)-(E) correspond to the crystal structures shown in Figure . For Ba-rich compositions (x < 0.5), it is more favorable for Ba/Sr disorder to occur on the intralayer site 2. Conversely, the interlayer site 1 is disorder in Sr-rich compositions (x > 0.5). The 50-50 alloy is energetically less stable. x in Ba With increasing Sr content (increasing x), we observe a nearlinear decrease in the unit cell volume consistent with Vegard's law (Figure ). Similarly, the cell parameter a exhibits an almost linear decrease with x, but cell parameters b and c change nonmonotonically (Figures ). Theoretically, we predict that the 50-50 alloy (x = 0.5) represents an inflection point in b and c as a function of x, which is consistent with experiments. The nonmonotonic changes and inflection in b and c are due to the site selectivity of Ba and Sr and the related structural distortions, as discussed above.