Octahedral Rotation Preferences in Perovskite Iodides and Bromides

Phase transitions in ABX₃ perovskites are often accompanied by rigid rotations of the corner-connected BX₆ octahedral network. Although the mechanisms for the preferred rotation patterns of perovskite oxides are fairly well recognized, the same cannot be said of halide variants (i.e., X = Cl, Br, or I), several of which undergo an unusual displacive transition to a tetragonal phase exhibiting in-phase rotations about one axis (a⁰a⁰c⁺ in Glazer notation). To discern the chemical factors stabilizing this unique phase, we investigated a series of 12 perovskite bromides and iodides using density functional theory calculations and compared them with similar oxides. We find that in-phase tilting provides a better arrangement of the larger bromide and iodide anions, which minimizes the electrostatic interactions, improves the bond valence of the A-site cations, and enhances the covalency between the A-site metal and Br^- or I^- ions. The opposite effect is present in the oxides, with out-of-phase tilting maximizing these factors.