THERMALLY-INDUCED DICHROISM IN CU2- STUDY OF THE THERMOCHROMIC PROPERTIES( DOPED [(CH3)(4)N]HGBR3 )

This work investigates the polarized Charge-transfer spectra associate d with Cu2+ impurities in [(CH3)(4)N]HgBr3:Cu2+ in the 10-300 K temper ature range. Attention is paid for correlating the optical spectra wit h the local structure around Cu2+. The spectra are compared with those obtained in the Cu2+ doped [(CH3)(4)N]CdBr3 and [(CH3)(4)N]MnBr3, and [(CH3)(4)N](2)HgBr4 and [(CH3)(4)N](2)CdBr4 crystals, where the Cu2ions form CuBr64- complexes of nearly D-4h symmetry and tetrahedral di storted CuBr42--complexes of D-2d symmetry, respectively. The presence of a strongly-polarized absorption band at 15900 cm(-1) in [(CH3)(4)N ]HgBr3:Cu2+ is interpreted in terms of a highly distorted CuBr42- comp lex. The enhancement of dichroism exhibited by this crystal with decre asing temperature is noteworthy. The analysis of the spectra reveals t hat this unusual thermal behaviour is mainly associated with orientati onal motions of the Cu2+ complex rather than with structural changes o f the complex.