Phonon Instabilities and Interactions near Solid-State Phase Transitions

Abstract

The recent observation of soft optic modes which trigger structural phase transitions of several different kinds leads naturally to the suggestion that the anomalous acoustic behavior near these phase transitions might arise from interactions between acoustic phonons and the soft optic modes. In this paper we explore this suggestion from both theoretical and experimental points of view. The concept of the soft mode is explained for both phonon and tunneling modes whose instabilities may occur at the center (q=0) or the boundary (q≈π/a) of the Brillouin zone. Illustrative experimental results from infrared absorption, light scattering, and neutron scattering techniques are presented. We review and compare several theories of the interaction between acoustic phonons and soft optic modes and cite appropriate experimental evidence, where available, for comparison. Among the crystals discussed are SrTiO3, KDP, KTaO3, and NH4Cl. We conclude that basically two types of optic-acoustic interactions are sufficient to describe most structure transitions. However, to date, a complete microscopic description in quantitative agreement with experiment, both above and below Tc, has yet to appear. Nevertheless, the explanation of acoustic anomalies in terms of soft optic mode behavior is essentially correct and will prove quite fruitful.