A simple theory of the temperature dependence of exchange interactions between E ions

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Abstract
The effect of Jahn-Teller coupling to a localized E vibrational mode on an orbital doublet, split by large non-cubic crystal-field components, is considered. The effective Hamiltonians of the weaker one- and two-ion interactions within the vibronic states are found and averaged over the thermal population due to rapid relaxations. In this way, one Hamiltonian with temperature-dependent parameters is obtained. Since the exchange between d ions depends strongly on their orbital states, the temperature dependence of the effective exchange parameter can be especially large. The theory can be satisfactorily applied to the interpretation of the experimentally known temperature behaviour of nearest-neighbour exchange interactions in K2CuCl42H2O.