SELF-CONSISTENT PHONON STUDIES OF A MODEL DIATOMIC FERROELECTRIC

A model of a diatomic crystal loosely patterned on tin telluride is de veloped. The crystal has the sodium chloride structure at high tempera tures, but displays a soft-mode instability as the temperature is lowe red. The lattice dynamics of the model is treated in the self-consiste nt phonon approximation. The free energy is found to show the behaviou r characteristic of a second-order phase transition, but the temperatu re of the transition is not the same as the temperature at which the f requency of the soft mode becomes zero. Additional contributions to th e anharmonic frequency shift, the bubble diagrams, are shown to shift the frequency of the soft mode so that it does go to zero at the trans ition temperature.