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 45° 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 (Fm3̅m), 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 CsInCl3. 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 CsInX3 are unique inorganic halide double perovskites because they are based on a single metal, with CsInBr3 just the third bromide after the mixed metal compounds Cs2AgBiBr6 and Cs2AgTlBr6.