Structural, electronic, and magnetic properties of nearly ideal Jeff= 12 iridium halides

Heavy transition metal magnets with Jeff=12 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 Jeff=12 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., K2IrCl6, K2IrBr6, (NH4)2IrCl6, and Na2IrCl6·6(H2O)] with Ir4+ electronic ground states exhibiting extremely small deviations from the ideal Jeff=12 limit. We also find ordered magnetic ground states for the three anhydrous systems, with single-crystal neutron diffraction on K2IrBr6 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 IrX6 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 Jeff=12 state. At the same time, we highlight undeniable deviations from this ideal state, even in clean materials with ideal IrX6 octahedra as inferred from the global cubic crystal structures.