Crystal electric field and structural study of J_eff = 1/2 K₂IrX₆ (X = Cl, Br)

Strong spin orbit coupling in 5d materials has been a focus of research due to the wide variety of exotic phases which can exist in these systems. Of particular interest are systems 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. Here we present detailed data on two Iridium halide materials, K₂IrX₆, X = Cl, Br, which have the antifluorite structure and the Ir atom inside separated halide octahedra. Powder X-ray scattering study show a newly observed structural transition in the Br compound at T_c=180K. Resonant inelastic X-ray scattering data and X-ray absorption data are combined with quantum chemistry calculations to study the crystal electric field levels, and show that these materials show record proximity to the ideal Jeff = ½ state at all temperatures down to T=5K.