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 A 2 M + M 3+ X 6, which form in the 3D elpasolite structure with alternating M + and M 3+ octahedra. Here, we report the ternary mixed-valent indium compounds CsInX 3 (X = Br, Cl). They adopt a tetragonal (space group I 4/ 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 5s 2 lone pair, which induces a 45° rotation of all In 3+ 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 In 3+ site. Temperature-dependent synchrotron X-ray diffraction reveals that both compounds form the cubic double perovskite structure at high temperature ( Fm 3̅ m ), thus demonstrating that the twist occurs on cooling from the melt. UV–vis spectroscopy reveals band gaps near 2.3–2.4 eV for the bromide and 3.0 eV for CsInCl 3 . High-pressure electrical transport measurements show significant enhancement in conductivity as pressure increases, indicating a narrowing of the band gap at high pressures. Temperature-dependent transport measurements at high pressure show uniformly semiconducting behavior, with no superconducting transition observed down to 2.4 K. The CsInX 3 are unique inorganic halide double perovskites because they are based on a single metal, with CsInBr 3 just the third bromide after the mixed metal compounds Cs 2 AgBiBr 6 and Cs 2 AgTlBr 6 .