Refractive indices and electro-optics at ferroelectric and structural phase

Abstract

Changes δn of refractive indices induced by structural phase transitions are considered. Two main categories of transitions are pointed out. In proper ferroelastics, morphic birefringence or the angle of indicatrix rotation are proportional to the order parameter η through a coefficient measurable by light scattering in the parent phase, while at most other transitions these are proportional to η2. Roles of η, deformation and hard lattice modes are treated separately and these are shown to give competitive contributions to δn in the latter case. Data on materials representing different categories of transitions are discussed.Changes of electro-optic coefficients, z, caused by structural transitions are treated either macroscopically or on the basis of a mode approach. In the latter, contributions to z of lattice vibrational modes are connected with changes of their Raman and infrared strengths. Both methods are used to discuss z in proper ferroelectric, improper ferroelectric and nonferroelectric phase transitions.