Spin-orbit coupling controlled ground states in the double perovskite iridates A₂BIrO₆ (A = Ba, Sr; B = Lu, Sc)

Iridates with the 5d⁴ electronic configuration have attracted recent interest due to reports of magnetically-ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a J = 0 electronic ground state for each Ir⁵⁺ ion. The major focus of prior research has been on the double perovskite iridates Ba₂YIrO₆ and Sr₂YIrO₆, where the nature of the ground states (i.e. ordered vs non-magnetic) is still controversial. Here we present neutron powder diffraction, high energy resolution fluorescence detected x-ray absorption spectroscopy (HERFD-XAS), resonant inelastic x-ray scattering (RIXS), magnetic susceptibility, and muon spin relaxation data on the related double perovskite iridates Ba₂LuIrO₆, Sr₂LuIrO₆, Ba₂ScIrO₆, and Sr₂ScIrO₆ that enable us to gain a general understanding of the electronic and magnetic properties for this family of materials. Our HERFD-XAS and RIXS measurements establish J = 0 electronic ground states for the Ir⁵⁺ ions in all cases, with similar values for Hund's coupling J_H and the spin-orbit coupling constant λ_SOC. Our bulk susceptibility and muon spin relaxation data find no evidence for long-range magnetic order or spin freezing, but they do exhibit weak magnetic signals that are consistent with extrinsic local moments. Our results indicate that the large λ_SOC is the key driving force behind the electronic and magnetic ground states realized in the 5d⁴ double perovskite iridates, which agrees well with conventional wisdom.