Electronic structure of Bi2Ir2O7 probed by resonant inelastic x-ray scattering at the oxygen K edge: Metallicity, hybridization, and electronic correlations

The electronic structure of metallic Bi2Ir2O7 has been investigated by a combination of soft x-ray absorption spectroscopy, x-ray emission spectroscopy (XES), and resonant inelastic x-ray scattering (RIXS) in the vicinity of the oxygen K edge. The O K-edge RIXS spectra are found to resemble the O K-edge XES spectra with resonating features but with an absence of any Raman modes, revealing the highly itinerant nature of this compound. The O K-edge response is compared with scalar relativistic band structure calculations within the local density approximation, which captures the main spectral characteristics. The RIXS spectra either display a localized or delocalized Ir 5d response, depending on whether the photon energy is tuned to the pre-edge or above. Our results uncover a significant coupling of O K-edge RIXS via the O 2p−Ir 5d hybridization to the partial density of states of the Ir 5d states. An alternative explanation of the Ir 5d−O 2p hybridization spectral structure in the optical region of the RIXS spectra is given in terms of Ir dd excitations. Our findings allow us to infer that the relative electronic correlations in the metallic Bi2Ir2O7 compound with sizable spin-orbit coupling are 0.06≤U/W≤0.22 and thus are in the moderate-to-weak regime. We found that the Jeff model, developed to understand the existence of insulating states in various iridate perovskites, is not applicable to Bi2Ir2O7, casting doubts on its applicability to pyrochlore iridates with moderate to weak electronic correlations. We argue that the strong O 2p−Ir 5d−Bi 6s/6p hybridization is responsible for the highly itinerant nature of this material. Our results establish the capability of O K-edge RIXS to identify the electronic ground state of 5d spin-orbit transition metal compounds and to qualitatively assess their degree of localization. Published by the American Physical Society 2025