Thermochromic properties of the ferroelectric Cu2+-doped [(CH3)(4)N]HgBr3:study of the temperature-induced dichroism

A new thermochromic material formed by doping Cu2+ into the ferroelectric ( TMA)HgBr3 (TMA = (CH3)(4)N) crystal is presented. The change of colour from green (room temperature) to red (low temperature) under polarized light ob servation is accompanied by a strong temperature-induced dichroism at low t emperature. The origin of these phenomena is investigated through the polar ized charge transfer spectra associated with Cu2+ complexes formed in the t itle compound, and their dependence on temperature in the 10-300 K range. A ttention is paid to correlating the optical spectra with the local structur e around Cu2+. The results are compared with those obtained in the tribromi de (TMA)Cd[Cu]Br-3 and (TMA)Mn[Cu]Br-3, and the tetrabromide (TMA)(2)Hg[Cu] Br-4 and (TMA)(2)Cd[Cu]Br-4 crystals, where Cu2+ forms CuBr64- complexes of nearly D-4h symmetry, and tetrahedral distorted CuBr42- complexes of Did s ymmetry, respectively. An interesting feature is the presence of a strongly polarized absorption band at 15 800 cm(-1) in (TMA)Hg[Cu]Br-3, which is re sponsible for the observed dichroism and thermochromism. The existence of t his band is associated with the formation of highly distorted CuBr42- compl exes in the monoclinic (TMA)HgBr3 host crystal. The enhancement of dichrois m 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 struct ural changes of the Cu2+ complex.