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

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 ${\theta }_{\mathrm{CW}}\simeq -43$ K, and the extrapolated saturation field of $H_s\simeq 87$ T. All these parameters are well reproduced \textit{ab initio} using $U_{\mathrm{eff}}=2.2$ 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.

9 pages

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