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Distinct shortcomings of individual halide perovskites for solar applications, such as restricted range of band gaps, propensity of ABX₃ to decompose into AX + BX₂, or oxidation of 2ABX₃ into A₂BX₆, have led to the need to consider alloys of individual perovskites such as [FA,Cs][Pb,Sn][Br,I]₃. This proposition creates a nontrivial material selection problem associated with a six-component structure, spanning a continuum of three sets of compositions (one for each sublattice) and requiring control of phase separation or ordering in each alloyed subfield. Not surprisingly, material and structure choices were made thus far mostly via trial-and-error explorations among a large number of arrangements. Here, we use ideas from solid-state theory of semiconductor alloys to analyze the behaviors of the canonical [FA,Cs][Pb,Sn]I₃ alloy system, where FA is formamidinium. Density functional calculations utilizing specially …