Thermochromic properties of the ferroelectric -doped : study of the temperature-induced dichroism

Abstract

A new thermochromic material formed by doping into the ferroelectric crystal is presented. The change of colour from green (room temperature) to red (low temperature) under polarized light observation is accompanied by a strong temperature-induced dichroism at low temperature. The origin of these phenomena is investigated through the polarized charge transfer spectra associated with complexes formed in the title compound, and their dependence on temperature in the 10-300 K range. Attention is paid to correlating the optical spectra with the local structure around . The results are compared with those obtained in the tribromide and , and the tetrabromide and crystals, where forms complexes of nearly symmetry, and tetrahedral distorted complexes of symmetry, respectively. An interesting feature is the presence of a strongly polarized absorption band at 15 800 in , which is responsible for the observed dichroism and thermochromism. The existence of this band is associated with the formation of highly distorted complexes in the monoclinic host crystal. The enhancement of dichroism exhibited by this crystal with decreasing temperature is noteworthy. The analysis of the spectra reveals that this unusual thermal behaviour can be explained in terms of thermally activated reorientations rather than structural changes of the complex.