Abstract
A theory is presented for the structural phase transitions occurring in crystals in which the ionic constituents are pseudo-Jahn-Teller ions with nearly twofold electronic degeneracy. We consider the case of a strong Jahn-Teller coupling and study the dynamic properties by means of a time-dependent density matrix which takes into account the correlation between the electronic and vibrational motions of a single ion. The results are compared with those obtained in a recent theory of Pytte and Stevens, in which single-ion terms are treated in a decoupling approximation.

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