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 ${\mathrm{Ir}}⁴⁺$ or ${\mathrm{Ir}}⁵⁺$ using hard x-ray photoelectron spectroscopy. 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 downfolding of the density-functional band structure results. We found that, regardless of the A and B cations, the ${\mathrm{A}}₂{\mathrm{BIrO}}₆$ iridates have essentially zero O $2p$ to Ir $5d$ charge-transfer energies. Hence double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long ranged, thereby hampering the materialization of the long-sought Kitaev physics. 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.