On the electronic ground state of two non-magnetic pentavalent honeycomb iridates

We investigate the electronic structure of two Ir${}^{5+}$ honeycomb iridates, Sr${}_3$CaIr${}_2$O${}_9$ and NaIrO${}_3$, by means of resonant x-ray techniques. We confirm that Sr${}_3$CaIr${}_2$O${}_9$ 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.