Jahn-Teller phase transitions; selection of the order parameter

A three-dimensional order parameter is proposed for Jahn-Teller phase transitions related to splitting of the ion E_g-level. A thermodynamic theory of these transitions is constructed on its basis. A selection rule of possible orderings is introduced that is based on the simultaneous entrance of the relevant order parameter into the composition of the mechanical and Jahn-Teller representations. Developed methods are approved for analysis of the structural transition in perovskites of the KCuF₃ type.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 51–55, December, 1989.