Interplay of Octahedral Rotations and Lone Pair Ferroelectricity in CsPbF₃

CsPbF₃ is the only experimentally synthesized ABF₃ fluoride perovskite with a polar ground state. We use CsPbF₃ as a guide in our search for rules to rationally design new ABX₃ polar fluorides and halides from first-principles and as a model compound to study the interactions of lone pairs, octahedral rotations, and A- and B-site driven ferroelectricity. We find that the lone pair cation on the B-site serves to stabilize a polar ground state, analogous to the role of lone pair cations on the A-site of oxide perovskites. However, we also find that the lone pair determines the pattern of nonpolar structural distortions, rotations of the PbF₆ octahedra, that characterize the lowest energy structure. This result is remarkable since rotations are typically associated with bonding preferences of the A-site cation (here Cs⁺), whereas the Pb²⁺ cation occupies the B site. We show that the coordination requirements of the A-site cation and the stereoactivity of the B-site lone pair cation compete or cooperate via the anionic displacements that accompany polar distortions. We consider the generalizability of our findings for CsPbF₃ and how they may be extended to the oxide perovskites as well as to the organic–inorganic hybrid halide perovskite photovoltaics

Efficient Microwave Method for the Oxidative Coupling of Phenols

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