Birefringent and electrooptical properties of (NH₄)₂BeF₄

Abstract

Experimental data are presented on the spontaneous birefringence and the linear electrooptical effect in the ferroelectric phase. A phenomenological description of these optical properties is given, which takes into account the improper nature of the ferroelectric transition. Two possible types of the ferroelectric phase changes, an improper and pseudoproper phase transition, are discussed. Satisfactory agreement is found between the theory of an improper ferroelectric phase transition and experimental data given here and by other authors. The pseudoproper model leads in our opinion to a fundamental inconsistence with our optical experiments.