Heavy transition metal magnets with $J_{eff}=\frac{1}{2}$ electronic ground states have attracted recent interest due to their penchant for hosting new classes of quantum spin liquids and superconductors. Unfortunately, model systems with ideal $J_{eff}=\frac{1}{2}$ states are scarce due to the importance of noncubic local distortions in most candidate materials. In this work, we identify a family of iridium halide systems [i.e., ${\mathrm{K}}_2\mathrm{Ir}{\mathrm{Cl}}_6$, ${\mathrm{K}}_2\mathrm{Ir}{\mathrm{Br}}_6$, ${\left({\mathrm{NH}}_4\right)}_2\mathrm{Ir}{\mathrm{Cl}}_6$, and ${\mathrm{Na}}_2\mathrm{Ir}{\mathrm{Cl}}_6·6\left({\mathrm{H}}_2\mathrm{O}\right)]$ with ${\mathrm{Ir}}^{4+}$ electronic ground states exhibiting extremely small deviations from the ideal $J_{eff}=\frac{1}{2}$ limit. We also find ordered magnetic ground states for the three anhydrous systems, with single-crystal neutron diffraction on ${\mathrm{K}}_2\mathrm{Ir}{\mathrm{Br}}_6$ revealing type-I antiferromagnetism. This spin configuration is consistent with expectations for significant Kitaev exchange in a face-centered-cubic magnet. This work establishes that incorporating isolated $\mathrm{Ir}{X}_6$ octahedra in materials, where $X$ is a halogen ion with a low electronegativity, is an effective design principle for realizing unprecedented proximity to the pure $J_{eff}=\frac{1}{2}$ state. At the same time, we highlight undeniable deviations from this ideal state, even in clean materials with ideal $\mathrm{Ir}{X}_6$ octahedra as inferred from the global cubic crystal structures.

• Received 2 September 2020
• Accepted 30 November 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.124407