The Electronic Structure and Properties of Negative Charge Transfer Compounds*

In high oxidation state oxides like the trivalent nickel oxides, tetravalent cobalt and iron oxides as well as the parent superconductors BaBiO3 and SrBiO3, the cation electron affinity for the formal valence taking oxygen to be 2- can result be larger than the oxygen ionization potential. This leads to a so-called negative charge transfer energy, where the cations are more accurately described with reduced valences and the oxygen states are self-doped with holes. As a consequence, one can have very different electronic structures and x-ray spectra than expected from the formal oxidation state picture. We demonstrate that with this in mind we can very well explain many of the properties and phases of the trivalent nickelates and tetravlent ferrates. Utilizing the negative charge transfer approach in cluster exact diagonalization calculations resolves a 25 year old puzzle concerning core level spectra of nickelates, and reveals interesting orbital polarization phenomena in strained nickelate and ferrate films.