MICROSCOPIC THEORY OF COOPERATIVE JAHN-TELLER EFFECT IN CSCUCL3

A Hamiltonian describing the cooperative Jahn-Teller (JT) effect is derived taking fully into consideration the symmetry both from the local and the crystal point of view. The coefficients of the JT interaction term in the Hamiltonian are expressed as a function of the local JT interaction constants. It is shown that the traditional description of the cluster CuCl6 as an approximate octahedron is incomplete. On the basis of the Hamiltonian obtained, the helical ordering of the Pseudo-spins which span the lowest electronic state of Cu2+ in the site symmetry D3d is derived. The tripling of the lattice periodicity and the helical arrangement of Cl- ion displacements of the compound follow as a result.