POLARIZED CHARGE-TRANSFER SPECTROSCOPY OF CU2-DIMENSIONAL [N(CH3)4]CDCL3 AND [N(CH3)4]CDBR3( IN DOPED ONE)

This work investigates the polarized charge-transfer (CT) spectra of t he Jahn-Teller distorted CuCl6(4-) and CuBr6(4-) complexes formed in C u2+-doped [N(CH3)4]CdCl3 and [N(CH3)4]CdBr3 CrYstals. The transition e nergies as well as the dominant polarization of the CT bands along the hexagonal c direction are explained in terms of rhombic D2h distortio ns. In the bromide complexes, the strong spin-orbit interaction of the Br- ligands leads to additional bands in the low-temperature spectra. Evidence of vibronic couplings to totally symmetric vibrations is fou nd from analysis of the temperature dependence of the band width. The influence of Cu2+ impurities in the lattice dynamics is studied throug h the variation undergone by different spectroscopic parameters in the 10-300 K range. Throughout this work, the high sensitivity of CT tran sitions for detecting the structural phase transitions exhibited by th ese crystals is demonstrated. In particular, the enhancement of the fi rst-order character of the ferroelectric phase transition in [N(CH3)4] CdBr3:Cu2+ at T(F) = 157 K, which has been associated with the presenc e of Cu2+ impurities is worth noting.