# Cubic symmetry and magnetic frustration on the $fcc$ spin lattice in K$_2$IrCl$_6$

## TL;DR

This study investigates the magnetic properties and exchange interactions in the cubic K$_2$IrCl$_6$ compound, revealing a complex interplay of Heisenberg and Kitaev interactions under high-symmetry conditions, supported by experimental and ab initio results.

## Contribution

It provides detailed experimental and theoretical analysis of exchange interactions in K$_2$IrCl$_6$, highlighting its potential as an ideal cubic model for studying magnetic frustration.

## Key findings

- Cubic symmetry persists down to 20 K.
- Antiferromagnetic Kitaev exchange is about half of the Heisenberg exchange.
- Weak second-neighbor coupling influences magnetic ground state.

## Abstract

Cubic crystal structure and regular octahedral environment of Ir$^{4+}$ render antifluorite-type K$_2$IrCl$_6$ a model fcc antiferromagnet with a combination of Heisenberg and Kitaev exchange interactions. High-resolution synchrotron powder diffraction confirms cubic symmetry down to at least 20 K, with a low-energy rotary mode gradually suppressed upon cooling. Using thermodynamic and transport measurements, we estimate the activation energy of $\Delta\simeq 0.7$ eV for charge transport, the antiferromagnetic Curie-Weiss temperature of $\theta_{\rm CW}\simeq -43$ K, and the extrapolated saturation field of $H_s\simeq 87$ T. All these parameters are well reproduced \textit{ab initio} using $U_{\rm eff}=2.2$ eV as the effective Coulomb repulsion parameter. The antiferromagnetic Kitaev exchange term of $K\simeq 5$ K is about one half of the Heisenberg term $J\simeq 13$ K. While this combination removes a large part of the classical ground-state degeneracy, the selection of the unique magnetic ground state additionally requires a weak second-neighbor exchange coupling $J_2\simeq 0.2$ K. Our results suggest that K$_2$IrCl$_6$ may offer the best possible cubic conditions for Ir$^{4+}$ and demonstrates the interplay of geometrical and exchange frustration in a high-symmetry setting.