We performed density functional theory calculations, first-principles molecular dynamics simulations, solid-state nuclear magnetic resonance (NMR), and X-ray powder diffraction (XRD) measurements aimed at investigating the possible phase stability of Cs+-rich FAxCs(1–x)PbI3, (0 ≤ x ≤ 0.5) mixed-cation materials as potential candidates for tandem solar cell applications. The atomistic origin for the energetic destabilization of the perovskite phase on the one hand is caused by the incorporation of a large cation (FA+) into the relatively small host lattice of γ-CsPbI3 and on the other hand is induced by the lower degree of distortion of the host lattice. These observations allow us to propose a new design principle for the preferential stabilization of the perovskite phase over the competing δ phase.