The 50-50 alloy (x = 0.5) is unique in that the Ba/Sr disorder must occur on both sites 1 and 2 because they are present in equal numbers in the structure. The site selectivity of Ba and Sr cannot be simultaneously accommodated for x = 0.5. Our calculated DG m (Figure ) suggests that it is possible to form the 50-50 alloy (DG m < 0) but energetically much less stable than compositions with x = 0.5 d and x = 0.5 + d , where d is a small positive number. As such, we predict that it will be challenging to experimentally synthesize the exact 50-50 alloy. For Ba-rich conditions (x = 0.00 -0.45), a disorder site selectivity for site 2 is observed while for Sr-rich (x = 0.55 -1.00) conditions, disorder prefers to occur at site 1.