On the charge transfer energy in iridates: a hard x-ray photoelectron spectroscopy study*

We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir⁴⁺ or Ir⁵⁺ using hard x-ray photoelectron spectroscopy (HAXPES). The experimental valence band spectra can be well reproduced using tight binding calculations including only the Ir 5d, O 2p and O 2s orbitals with parameters based on the down-folding of the density-functional band structure results. We found that regardless the A and B cations, the A₂BIrO₆ iridates have essentially zero O 2p to Ir 5d charge transfer energies. They are extremely covalent systems with the consequence is that the magnetic exchange interactions become very long-ranged, thereby hampering the materialization of the Kitaev model. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions .