Perovskites with a Twist: Strong In1+ Off-centering in the Mixed-Valent CsInX3 (X = Cl, Br)

The search for new halide perovskites has recently expanded to the double perovskites A2M+M3+X6, which form in the 3D elpasolite structure with alternating M+ and M3+ octahedra. Here, we report the ternary mixed-valent indium compounds CsInX3 (X = Br, Cl). They adopt a tetragonal (space group I4/m) structure which is a derivative of the charge-ordered double perovskite but with a twisted chain of octahedra along the c-axis. This twist is a result of the off-centering at the In+ site to accommodate its stereochemically active 5s2 lone pair, which induces a 45o rotation of all In3+ octahedra in the chain. This is a non-cooperative rotation that creates pentagonal pyramids in the structure and induces significant disorder, with extensive twinning which results in partial occupation of different rotations of the same In3+ site. Temperature-dependent synchrotron X-ray diffraction reveals that both compounds form the cubic double perovskite structure at high temperature (Fm-3m), thus demonstrating that the twis...