Exchange interactions between orbitally degenerate ions in Ti2 X 9 3- (X=Cl, Br)

Abstract
An effective hamiltonian ℋeff is developed for the treatment of exchange interactions in a pair of transition metal ions with orbitally degenerate ground states. In contrast to a Heisenberg-Dirac-van Vleck (HDvV) hamiltonian ℋeff retains a partial degeneracy of singlet and triplet pair states. The hamiltonian is applied to Ti2 X 9 3- (X=Cl, Br) dimers. The gross experimental features of Cs3Ti2Cl9 and Rb3Ti2Br9 are well reproduced by the theoretical model. The coupling is strongly antiferromagnetic and the exchange parameters can be rationalized by an approximate MO calculation.