Crystal field and superposition model analysis for high-spin Fe2+ and Fe4+ ions in YBa2(Cu1-xFex)3O7- delta

Abstract

Crystal field energy levels of Fe2+ and Fe4+ ions at several possible sites in YBa2(Cu1-xFex)3O7- delta namely, Cu(1) planar, pyramidal and octahedral sites, as well as Cu(2) sites, have been investigated. The superposition model formulae are developed to relate the crystal field parameters Bka with the available crystal structure parameters. The parameters Bka are calculated for Fe2+ and Fe4+ ions using the most likely values of the intrinsic parameters determined for crystal structures similar to that of YBa2Cu3O7. The crystal field energy levels and the ground states are then determined for Fe2+ and Fe4+ ions at each site. The superposition model enables the authors to also study the dependence of Bka and energy levels on the concentration (x) of Fe ions in YBa2(Cu1-xFex)3O7- delta . The ground state for most of the cases considered appears to be an orbital singlet with high spin S=2 and well separated from the excited 5D orbital levels due to the strong orthorhombic crystal field. This justifies application, in a subsequent paper, of the spin Hamiltonian analysis to gain further insight into the effects of Fe2+ and Fe4+ ions on the properties of high-Tc 123 superconductors.