Evaluating covalency using resonant inelastic x-ray scattering spectral weights: Silver fluorides versus cuprates

We investigate the electronic structure of AgF2, AgFBF4, AgF, and Ag2O using x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) at the Ag L3 edge. XAS results were compared with density functional theory computations of the spectra, allowing an identification of main features and an assessment of the theoretical approximations. Our RIXS measurements reveal that AgF2 exhibits charge-transfer (CT) excitations and dd excitations, analogous to those observed in La2CuO4. We propose to use the ratio of dd to CT spectral weight as a measure of the covalence of the compounds and provide explicit equations for the weights as a function of the scattering geometry for crystals and powders. The measurements at the metal site L3 edge and previous measurement in the ligand K edge reveal a striking similarity between the fluoride and cuprate materials, with fluorides somewhat more covalent than cuprates. These findings support the hypothesis that silver fluorides are an excellent platform to mimic the physics of cuprates, providing a promising avenue for exploring high-Tc superconductivity and exotic magnetism in quasi-two-dimensional (AgF2) and quasi-one-dimensional (AgFBF4) materials.