Phenomenological lattice-dynamical theory of ferroelasticity

Abstract

A phenomenological lattice-dynamical investigation is made into ferroelasticity and some of its associated phenomena, such as optical birefringence and elastooptic effect, in ferroelastic crystals that are nonmagnetic and may or may not be ferroelectric at the same time. Two different types, provisionally called optical type and elastic type, are conceivable for ferroelastics. Ferroelastics of the optical type are optically soft and elastically hard. They have a special optical mode of lattice vibration that is Raman-active and lowers rapidly in eigenfrequency as temperature falls toward the Curie point. Ferroelastics of the elastic type are elastically soft and optically hard. In them, a certain elastic stiffness when the coordinates of all lattice vibration modes with zero wave number are fixed diminishes rapidly as temperature falls toward the Curie point. The investigation is carried out for both these different types of ferroelastics, which are found to behave similarly in respects proper to ferroelasticity and to behave differently in not a few other important respects.