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

TL;DR

This study investigates the structural, electronic, and magnetic properties of K$_2$IrBr$_6$, revealing temperature-induced symmetry reductions and their effects on magnetic ordering and electronic interactions.

Contribution

It provides detailed insights into the temperature-dependent symmetry changes and electronic effects in K$_2$IrBr$_6$, combining experimental and ab initio methods.

Findings

Crystal symmetry reduces from cubic to tetragonal below 170 K and to monoclinic below 122 K.

The Néel temperature is 11.9 K, higher than in similar chloride compounds.

Replacing Cl with Br weakens electronic correlations and enhances magnetic interactions.

Abstract

Crystal structure, electronic state of Ir$^{4+}$, and magnetic properties of the antifluorite compound K$_2$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 non-cubic crystal-field splitting $\Delta$ measured by RIXS. Non-monotonic changes in $\Delta$ are ascribed to the competing effects of the tilt, rotation, and deformation of the IrBr$_6$ octahedra as well as tetragonal strain on the electronic levels of Ir$^{4+}$. The N\'eel temperature of $T_N=11.9$ K exceeds that of the isostructural K$_2$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.