Near Ideal Realization of the Jeff = ½ State in Ir Antifluorite Compounds.*

Strong spin orbit coupling in 5d materials can lead to a particular type of Mott physics. For a 5d atom with octahedral coordination, the triply degenerate t_2g state will split into the occupied J_eff = 3/2 and the partially occupied J_eff = ½ states. It is this J_eff = ½ state that can host a large range of exotic phases such as quantum spin liquids and superconductivity. However it is debated how close many of the currently studied J_eff = ½ Ir compounds are to this limit. In many cases, non-cubic crystal fields are of the same energy scale as the spin orbit coupling and cause the J_eff = ½ and the J_eff = 3/2 states to mix. We present data on a several Iridium halide materials, M₂IrX₆ M = K, Na and NH₃ and X = Cl, Br, which have the antifluorite structure and the Ir atom inside separated halide octahedra. We present resonant inelastic X-ray scattering and X-ray absorption data that show record low splitting of the t_2g orbitals, suggesting that these materials are a better realization of the Jeff = ½ state than any previously studied material. We combine these results with neutron scattering and muon spin spectroscopy to additionally explore their magnetic states.