Negative Charge Transfer Energy in Correlated Compounds

In correlated compounds containing cations in high formal oxidation states\n(assigned by assuming that anions attain full valence shells), the energy of\nligand to cation charge transfer can become small or even negative. This yields\ncompounds with a high degree of covalence and can lead to a self-doping of\nholes into the ligand states of the valence band. Such compounds are of\nparticular topical interest, as highly studied perovskite oxides containing\ntrivalent nickel or tetravalent iron are negative charge transfer systems, as\nare nickel-containing lithium ion battery cathode materials. In this report, we\nreview the topic of negative charge transfer energy, with an emphasis on plots\nand diagrams as analysis tools, in the spirit of the celebrated Tanabe-Sugano\ndiagrams which are the focus of this Special Topics Issue.\n