Abstract

Starting from the chemical principle of charge neutrality, one can make assumptions regarding the oxidation states of elements in compounds based on transition metals, particularly in the insulating states. However, a more accurate method for estimating oxidation states is the bond valence sum model, which utilizes experimental crystal structures. These estimates sometimes yield fractional oxidation states, which are verified by experimental techniques like resonant X-ray scattering (RXS) that employ empirical arguments. Despite its validity, the interpretation of RXS experiments using numerical methods often indicates minimal charge disproportionation, which contradicts the estimated oxidation states. Additionally, RXS predicts charge disproportionation in metallic systems, such as the example of 2H-AgNiO2. In this presentation, I will provide a concise explanation of the fundamentals of the RXS method. Furthermore, employing the dynamical mean field theory (DMFT), I will introduce an alternative perspective on chemically estimated oxidation states, using compounds like NdNiO3, CaFeO3, LaMnO3 and BiMnO3, as examples. This new alternative perspective suggests that the presence of “charge disproportionation” in materials is synonymous with the emergence of unique states of matter characterized by site or orbital selectivity. In these novel states, we observe intriguing phenomena such as non-equivalent crystallographic sites exhibiting characteristics of both Mott and band-like insulators, or within the same crystallographic site, the coexistence of metallic and insulating orbitals.