Electronic Ground State of Two Nonmagnetic Pentavalent Honeycomb Iridates

We investigate the electronic structure of two Ir^5+ honeycomb iridates, Sr_3CaIr_2O_9 and NaIrO_3, by means of resonant x-ray techniques. We confirm that Sr_3CaIr_2O_9 realizes a large spin-orbit driven non-magnetic J = 0 singlet ground state despite sizable tetragonal distortions of Ir coordinating octahedra. On the other hand, the resonant inelastic x-ray spectra of NaIrO_3 are drastically different from expectations for a Mott insulator with octahedrally coordinated Ir^5+. We find that the data for NaIrO_3 can be best interpreted as originating from a narrow gap non-magnetic S = 0 band insulating ground state. Our results highlight the complex role of the ligand environment in the electronic structure of honeycomb iridates and the essential role of x-ray spectroscopy to characterize electronic ground states of insulating materials.