Charge Transfer and $d-d$ excitations in AgF$_{2}$.

Charge transfer (CT) insulators are the parent phase of a large group of today's unconventional high temperature superconductors. Here we study experimentally the interband excitations of the electronic structure of a CT insulator in the form of silver fluoride, AgF$_2$, which has been proposed as an excellent analogue of oxocuprates. Optical conductivity and resonant inelastic X-Ray scattering (RIXS) on AgF$_2$ polycrystalline sample show a close similarity with that being measured on undoped La$_2$CuO$_4$ while the former show a larger CT gap of approximately 3.4 eV, $d-d$ excitations are nearly at the same energy in both materials. DFT and exact computations in a small cluster show that AgF$_2$ is more covalent than cuprates despite having a larger fundamental gap. In addition, our computations show that AgF$_2$ is at the verge of a charge transfer instability. The overall resemblance of our data on AgF$_2$ to those published before on its cuprate equivalent La$_2$CuO$_4$ suggests that the underlying CT insulator physics is the same, while AgF$_2$ could also benefit from a proximity to a charge density wave phase as in BaBiO$_3$. Therefore, our work provides a compelling support to the future use of fluoroargentates for the growth of novel high-temperature superconductors.