Strength of correlations in a silver-based cuprate analog

AgF2 has been proposed as a cuprate analog, which requires strong correlation and marked covalence. On the other hand, fluorides are usually quite ionic, and 4d transition metals tend to be less correlated than their 3d counterparts, which calls for further scrutiny. We combine valence band photoemission and Auger-Meitner spectroscopy of AgF and AgF2 together with computations in small clusters to estimate values of the Ag 4d Coulomb interaction U4d and charge-transfer energy Δpd. Based on these values, AgF2 can be classified as a charge-transfer correlated insulator according to the Zaanen-Sawatzky-Allen classification scheme. Thus, we confirm that the material is a cuprate analog from the point of view of correlations, suggesting that it should become a high-temperature superconductor if metallization is achieved by doping. We present also a computation of the Hubbard U in density functional “+U” methods and discuss its relation to the Hubbard U in spectroscopies.