Influence of crystal soft mode on phase transitions in impurity systems

Abstract

The role of the soft mode of the host crystal in a phase transition in the system of reorienting impurity centers is discussed. The energies of the vibronically active crystalline vibrations in the para-phases of the impurity subsystem and in the ordered one are different due to their coupling with the impurity electronic states. This energy difference together with the interimpurity interaction determines the temperature of the phase transition. The soft vibration promotes the transition of the impurity subsystem to a new phase, but the lowering of its frequency below a certain value can result in reduction of the transition temperature.