Testing the validity of the strong spin-orbit-coupling limit for octahedrally coordinated iridates in a model system Sr₃CuIrO₆

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The electronic structure of Sr₃CuIrO₆, a model system for the 5d Ir ion in an octahedral environment, is studied through a combination of resonant inelastic x-ray scattering (RIXS) and theoretical calculations. RIXS spectra at the Ir L₃-edge reveal an Ir t₂g manifold that is split into three levels, in contrast to the expectations of the strong spin-orbit-coupling limit. Effective Hamiltonian and ab inito quantum chemistry calculations find a strikingly large non-cubic crystal field splitting comparable to the spin-orbit coupling, which results in a strong mixing of the jeff=1/2 and jeff=3/2 states and modifies the isotropic wavefunctions on which many theoretical models are based.