Commensurate and incommensurate structural phase transitions in a Jahn-Teller system

Abstract

The possibility of structural phase transitions from a disordered phase to a modulated commensurate or incommensurate phase induced by the cooperative Jahn-Teller effect in an E(X) epsilon system is studied. The results obtained are in qualitative agreement with the structural phase transitions observed in CsCuCl3 and K2PbCu(NO2)6. Anharmonic terms and second-order Jahn-Teller coupling terms are found to play an essential role. Umklapp terms in the fourth-order anharmonicity provide the driving mechanism for the first-order transition from the incommensurate phase to the commensurate phase.