Cubic crystal structure and regular octahedral environment of ${\mathrm{Ir}}^{4+}$ render antifluorite-type ${\mathrm{K}}_2{\mathrm{IrCl}}_6$ 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 $\mathrm{\Delta }\simeq 0.7\mathrm{eV}$ for charge transport, the antiferromagnetic Curie-Weiss temperature of ${\theta }_{\mathrm{CW}}\simeq -43\mathrm{K}$, and the extrapolated saturation field of $H_s\simeq 87\mathrm{T}$. All these parameters are well reproduced ab initio using $U_{\mathrm{eff}}=2.2\mathrm{eV}$ as the effective Coulomb repulsion parameter. The antiferromagnetic Kitaev exchange term of $K\simeq 5\mathrm{K}$ is about one half of the Heisenberg term $J\simeq 13\mathrm{K}$. 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 $J_2\simeq 0.2\mathrm{K}$. Our results suggest that ${\mathrm{K}}_2{\mathrm{IrCl}}_6$ may offer the best possible cubic conditions for ${\mathrm{Ir}}^{4+}$ and demonstrates the interplay of geometrical and exchange frustration in a high-symmetry setting.

• Received 4 March 2019
• Revised 5 April 2019

DOI:https://doi.org/10.1103/PhysRevB.99.144425