Theory of high-spin⇄low-spin transitions in transition metal compounds induced by cooperative molecular distortions and lattice strains

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Abstract
In the previous paper [J. Chem. Phys. 70, 4199 (1979)] the high-spin ⇄ low-spin transitions were studied by using the model based on the static treatment of intramolecular distortion, in which the discontinuous spin transitions may be induced by the Jahn–Teller effect. In the present paper the intramolecular distortions are treated as dynamical variables and the intermolecular coupling between the intramolecular distortions is taken into account. It is shown that the results for the present treatment are equivalent to those for the previous treatment except that the meanings of the parameters are different between the two treatments. The effect of the coupling between transition metal ions and a lattice strain on the spin transition is also studied. The lattice strain may drive the spin transition accompanying the cooperative intramolecular distortion, even when the intramolecular coupling is not so strong that it can singly induce the spin transition. The discontinuous change of the lattice strain then takes place together with an occurrence of the cooperative intramolecular distortion. When there is no cooperative intramolecular distortion, the lattice strain varies continuously with temperature and the spin state also varies continuously, only that the discontinuous thermal change of the lattice strain may occur in a very narrow range of the coupling strength.