Temperature- and Pressure-Dependent Symmetry-Breaking Transitions in K ₂ IrCl ₆

A variable temperature X-ray total-scattering study of K₂IrCl₆ reveals compelling evidence for local symmetry breaking in this material. While the average crystal structure remains cubic down to 11 K, consistent with earlier reports, large anisotropic chloride displacements suggest short-range distortions of the IrCl₆ octahedra. Pair distribution function analysis confirms that the local structure is better described by a monoclinic <i>P</i>2₁/<i>n</i> model featuring a mix of in-phase and out-of-phase octahedral tilts. This behavior mirrors observations in related K₂MX₆ halides, where thermally driven cubic-to-monoclinic transitions occur. High-pressure synchrotron measurements further reveal two structural transitions: cubic <i>Fm</i>3̅<i>m</i> to tetragonal <i>P</i>4/<i>mnc</i> at 12.0 GPa, and tetragonal to monoclinic <i>P</i>2₁/<i>n</i> at 15.1 GPa. Both transitions are reversible on decompression. Lattice parameter refinements indicate anisotropic compression with the bulk modulus increasing dramatically from 23 GPa in the cubic phase to 121 GPa in the monoclinic structure. These results demonstrate that both temperature reduction and applied pressure drive K₂IrCl₆ toward lower-symmetry phases. Overall, this study provides the first direct local-structure evidence of symmetry breaking in K₂IrCl₆ and highlights the complex interplay among pressure, temperature, and local structure in vacancy-ordered double perovskites.