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Crystal structure, electronic state of Ir$^{4+}$, and magnetic properties ofthe antifluorite compound K$_2$IrBr$_6$ are studied using high-resolutionsynchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS),thermodynamic and transport measurements, and ab initio calculations. Thecrystal symmetry is reduced from cubic at room temperature to tetragonal below170 K and eventually to monoclinic below 122 K. These changes are tracked bythe evolution of the non-cubic crystal-field splitting $\Delta$ measured byRIXS. Non-monotonic changes in $\Delta$ are ascribed to the competing effectsof the tilt, rotation, and deformation of the IrBr$_6$ octahedra as well astetragonal strain on the electronic levels of Ir$^{4+}$. The N\'eel temperatureof $T_N=11.9$ K exceeds that of the isostructural K$_2$IrCl$_6$, and themagnitude of frustration on the fcc spin lattice decreases. We argue that thereplacement of Cl by Br weakens electronic correlations and enhances magneticcouplings.

Toward cubic symmetry for Ir$^{4+}$: structure and magnetism of antifluorite K$_2$IrBr$_6$