Electronic ground state of two nonmagnetic pentavalent honeycomb iridates

We investigate the electronic structure of two ${\mathrm{Ir}}^{5+}$ honeycomb iridates, ${\mathrm{Sr}}_{3}{\mathrm{CaIr}}_{2}{\mathrm{O}}_{9}$ and ${\mathrm{NaIrO}}_{3}$, by means resonant x-ray techniques. confirm that realizes a large spin-orbit driven nonmagnetic ${J}_{\mathrm{eff}}=0$ singlet ground state despite sizable tetragonal distortions Ir coordinating octahedra. However, inelastic spectra ${\mathrm{NaIrO}}_{3}$ are drastically different from expectations for Mott insulator with octahedrally coordinated ${\mathrm{Ir}}^{5+}$. find data can be best interpreted as originating narrow gap $S=0$ band insulating state. Our results highlight complex role ligand environment in iridates essential spectroscopy to characterize states materials.