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 ? ε _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_ε symmetry, the local Jahn-Teller effect has Q_θ symmetry (? = 0). Phase transitions in dependence on the Cu(II) concentration are analysed in detail for oxidic ordered perovskites Ba(Sr)₂Zn(Ni)_1-x Cu_xW(Te)O₆, 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_ε-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.