Generalized superexchange theory of fast energy transfer, cooperative luminescence, and magnetic exchange interactions in exchange-coupled pairs of lanthanide ions

A generalized theory of superexchange interaction between lanthanide ions is presented which describes both magnetic interactions and optical phenomena in exchange-coupled pairs of lanthanide ions. This theory is based on a new approach to the problem of superexchange interaction between metal ions in insulators, which is specially adapted for a complicated electronic structure of f(N) shell of lanthanide ions. A general procedure for calculations of the effective superexchange Hamiltonian for exchange-coupled Fair of exited lanthanide ions is developed. Estimations of the efficiency of the superexchange mechanism in optical effects show that this mechanism dominates in energy transfer processes and intensities of cooperative optical transitions in exchange-coupled pairs of lanthanide ions. The origin of high anisotropy of magnetic exchange interactions between lanthanide ions is quantitatively analyzed for the first time on the basis of detailed spin-Hamiltonian calculations for the 90 and 180 degrees f(1)-f(1) superexchange. Further developments of this theory and its applications to actual lanthanide compounds are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.