IMPURITY MODEL FOR MIXED-VALENT MN3+ MN4+ IONS/

Intermediate valent tri-and tetravalent manganese ions play an importa nt role in LaMnO3-based systems. We consider a Mn impurity with five o rbitals in cubic symmetry which hybridize with conduction electrons. T he exchange interaction in the d shell maximizes the impurity spin. We study the valence of the Mn impurity as a function of the splitting o f the e(g) to t(2g) orbitals in zero magnetic field and for the totall y spin-polarized state. The lifting of the degeneracy of the e(g) leve ls due to a small quadrupolar field, related to the Mn-O bond length o r a Jahn-Teller effect. is also investigated. Possible implications on the magnetoresistance are discussed.