Phonon Instabilities and Interactions near Solid-State Phase Transitions

Abstract

The rôle of phonon instabilities (soft modes) in several types of solid‐state structural phase transitions has recently been demonstrated by inelastic light‐scattering experiments. The symmetry of the soft phonon mode relates the crystal structures above and below T_c, and the temperature dependence of its frequency is associated with various crystal properties near T_c. Examples of both Brillouin zone‐center (q = 0) and zone‐boundary (q≠0) soft modes are presented. The interactions between optic and acoustic phonons respOnsible for sound‐velocity anomalies near T_c are discussed for several classes of phase transitions (ferroelectric, order‐disorder, etc.). While emphasis is placed on phase transitions in crystals of the perovskite structure (e.g., ${\mathrm{SrTiO}}_3$ ), some attention is also given to interesting transitions in other types of crystals.