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 (Fm3̄ 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 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.