Magnetic exchange interaction in a pair of orbitally degenerate ions: Magnetic anisotropy of [Ti2Cl9](-3)

The theory of the kinetic exchange in a pair of orbitally degenerate ions d eveloped by the authors [J. Phys. Chem. A 102, 200 (1998)] is applied to th e case of face-shared bioctahedral dimer (overall D-3h-symmetry). The effec tive kinetic exchange Hamiltonian is found for a T-2(2)-T-2(2) system takin g into account all relevant transfer pathways and charge-transfer crystal f ield states. The influence of different transfer integrals involved in the kinetic exchange on the energy pattern and magnetic properties of the syste m is examined. The role of other related interactions (trigonal crystal fie ld, spin-orbit coupling) is also discussed in detail. Using the pseudoangul ar momentum representation and the technique of the irreducible tensor oper ators of R-3-group we give a general outlook on the nontrivial symmetry pro perties of the effective Hamiltonian for the D-3h-pair, and on the magnetic anisotropy arising from the orbital interactions specific for the case of orbital degeneracy. The magnetic properties of the binuclear unit [Ti2Cl9]( -3) in Cs3Ti2Cl9 are discussed with a special emphasis on the magnetic anis otropy experimentally observed in this system. The existing exchange models for [Ti2Cl9](-3) and the concept of the effective Hamiltonian are discusse d in the context of the present study. (C) 2001 American Institute of Physi cs.