Phase Transitions and Soft Librational Modes in Cubic Crystals

Landau's theory of second-order phase transitions and the theory of lattice dynamics are combined in a presentation of an approach to the understanding of the crystallographic phase transitions of complex fcc crystals. The approach is applied to the antifluorite-type crystal K₂ReCl₆, which exhibits four phase transitions. The results of a series of crystallographic and spectrographic experiments are described, and are interpreted in light of the developed theory. Finally, a model based on the theory is constructed and is shown to describe the spectral observations. The resultant model leads to the conclusion that two of the phase transitions are driven by a single branch of a "soft" phonon.