Four-centre Jahn-Teller effect

In this work the four-centre Jahn-Teller effect and its influence on the structure and magnetic properties of Cu4OX 6 L 4 copper tetraclusters is investigated. The resulting adiabatic potential in the subspace of E vibrations is qualitatively similar in shape to the E-e problem. In the strong Jahn-Teller coupling limit, when the splitting of the degenerate electronic states of the Cu2+ ions is large enough, the Heisenberg spin hamiltonian is valid for the exchange interaction. The spin energy spectrum is determined, taking account of nuclear dynamics, and the temperature dependence of the effective magnetic moment μeff is evaluated. The exchange and vibronic parameter values, producing different types of dependencies μeff(T), including the anomalous, non-monotonic one, are discussed. The effect of the spin-orbit interaction on the temperature dependence of the μeff values is determined. For a number of copper tetraclusters, good agreement is obtained between the theoretical curves μeff(T), resulting from the vibronic model in the strong Jahn-Teller coupling case, with the experimental data. It is shown that the X-ray, E.P.R. and μeff(T) experimental data for Cu4OCl6(TPPO)4 tetraclusters can be understood within the above vibronic approach with a given relation between the crystal field, tunnelling and exchange parameters.