Jahn-Teller-type phase transitions in oxide ceramics and other solids with Cu(II) - from the viewpoint of a solid state chemist

After a short outline of the vibronic features of the local Jahn-Teller effect (E(g) x epsilon(g) coupling) the cooperative aspects - in particular the various order patterns of the elongated octahedra in Cu(II) solids - are reviewed. While the elastic interactions can be described by energy terms of Q(epsilon) symmetry, the local Jahn-Teller effect has Q(theta) symmetry (phi = 0). Phase transitions in dependence on the Cu(II) concentration are analysed in detail for oxidic ordered perovskites Ba(Sr)(2)Zn(Ni)(1-x)Cu(x)W(Te)O(6), which exhibit ferrodistortive order patterns connected with first order phase transitions at high x-values. At lower x-values at the short-range order scale elastic Q(epsilon)-type strains prevail as in the cases of antiferrodistortive order, leading to continuous phase transitions. Here, the cluster distribution has been successfully analysed by EPR spectroscopy.