Temperature Variations of Optical Indicatrix and Birefringence in Ferroelectric CsH₂PO₄

Abstract

The principal values and the rotation angle of the optical indcatrix and the optical birefringence of CsH₂PO₄ have been measured at 589 nm as a function of temperature both in the paraelectric and the ferroelectric phases. The principal values are found to be n_α=1.509, n_β=1.515 and n_γ=1.533 at room temperature, where n_α is the value parallel to the b-axis. Rotation of the optical indicatrix with temperature about the b-axis is within 1° between 100 K and 300 K. The spontaneous birefringence determined for (010) plate by the usual procedure reaches a maximum at about 15 K below T_c, then becomes negative with decreasing temperature. Similar behavior is found for (100) plate. It is found that these anomalous behaviors of the spontaneous birefringence are caused by the similar anomalous behavior in the spontaneous strain through the elastooptic coupling.