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

TL;DR

This study uses resonant x-ray techniques to analyze the electronic ground states of two honeycomb iridates, revealing a non-magnetic J=0 state in Sr3CaIr2O9 and a band insulator state in NaIrO3, emphasizing ligand effects.

Contribution

It provides the first detailed spectroscopic characterization of these iridates, clarifying their electronic ground states and the influence of ligand environment on their properties.

Findings

Sr3CaIr2O9 has a non-magnetic J=0 ground state despite distortions.

NaIrO3 exhibits a narrow gap non-magnetic S=0 band insulating state.

Ligand environment critically affects the electronic structure of honeycomb iridates.

Abstract

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.