ORBITAL ORDERING IN CUGEO3

The combined effect of the ligand crystal field and the exchange inter action on the Cu2+ cation in CuGeO3 is examined. It is shown that, if the magnitude of the exchange interaction exceeds the splitting of the energy levels of the d(x2-y2) and d(xy) orbitals, then an alternate f illing of the d orbitals along a chain (orbital ordering) is possible. This effect creates an antiferromagnetic interaction between Cu2+ pai rs in 90 degrees exchange and a doubling of the lattice period. A Jahn -Teller pseudoeffect causes singletization of the ground state of the antiferromagnetic chain of Cu2+ spins. (C) 1998 American Institute of Physics. [S1063-7834(98)02609-4].