Cation spin and superexchange interaction in oxide materials below and above spin crossover under high pressure

We derived simple rules for the sign of 180◦ superexchange interaction based on the multielectron calculations of the superexchange interaction in the transition metal oxides that are valid both below and above spin crossover under high pressure. The superexchange interaction between two cations in dn configurations is given by a sum of partial contributions related to the electron-hole virtual excitations to the different states of the dn+1 and dn−1 configurations. Using these rules, we have analyzed the sign of the 180◦ superexchange interaction of a number of oxides with magnetic cations in electron configurations from d2 until d8: the iron, cobalt, chromium, nickel, copper, and manganese oxides with increasing pressure. The most interesting result concerns the magnetic state of cobalt and nickel oxides CoO, Ni2O3 and also La2CoO4, LaNiO3 isostructural to well-known high-TC and colossal magnetoresistance materials. These oxides have a spin 1 2 at the high pressure. Change of the interaction from antiferromagnetic below spin crossover to ferromagnetic above spin crossover is predicted for oxide materials with cations in d5(FeBO3 ) and d7(CoO) configurations, while for materials with the other dn configurations spin crossover under high pressure does not change the sign of the 180◦ superexchange interaction