Towards cubic symmetry for \nIr4+\n: Structure and magnetism of the antifluorite \nK2IrBr6

Abstract

Crystal structure, electronic state of ${\\mathrm{Ir}}^{4+}$, and magnetic properties of the antifluorite compound ${\\mathrm{K}}_{2}{\\mathrm{IrBr}}_{6}$ are studied using high-resolution synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS), thermodynamic and transport measurements, and ab initio calculations. The crystal symmetry is reduced from cubic at room temperature to tetragonal below 170 K and eventually to monoclinic below 122 K. These changes are tracked by the evolution of the noncubic crystal-field splitting $\\mathrm{\\ensuremath{\\Delta}}$ measured by RIXS. Nonmonotonic changes in $\\mathrm{\\ensuremath{\\Delta}}$ are ascribed to the competing effects of the tilt, rotation, and deformation of the ${\\mathrm{IrBr}}_{6}$ octahedra as well as tetragonal strain on the electronic levels of ${\\mathrm{Ir}}^{4+}$. The N\\ eel temperature of ${T}_{N}=11.9$ K exceeds that of the isostructural ${\\mathrm{K}}_{2}{\\mathrm{IrCl}}_{6}$, and the magnitude of frustration on the fcc spin lattice decreases. We argue that the replacement of Cl by Br weakens electronic correlations and enhances magnetic couplings.