Optical and electro-optical properties of lithium thallium tartrate monohydrate

Abstract

The electro-optic coefficients rij and birefringence components of lithium thallium tartrate (LTT) single crystals have been measured in the 5-300 K temperature range, as also have the refractive indices at room temperature. The coefficients r41, ra and rb behave in an anomalous way in the vicinity of Tc approximately=12 K where LTT undergoes a phase transition; below Tc, r41 reaches a high value of 500 pm V-1. Their behaviour can be explained by considering the transition to be of a proper ferroelectric-proper ferroelastic nature, triggered by a balance between the values of clamped susceptibility, elastic stiffness and piezoelectric coefficient, as suggested earlier. A model of built-in strain is employed to interpret the existence of the symmetry-breaking coefficients ra and rb in the symmetric phase. The measured optical properties are used to re-examine the possibility of distinguishing domains in polarised light.