CHARGE-TRANSFER SPECTRA AND DYNAMICS OF CUBR42- IN [N(CH3)(4)]2CDBR4-CU2+ CRYSTALS - A NEW FIRST-ORDER PHASE-TRANSITION AT T-C2=20 K

The charge-transfer electronic structure of CuBr42- in [N(CH3)(4)]2CdBr4 crystals is investigated through polarized optical absorption spectroscopy. The transition energy and the band polarization are explained in terms of the Jahn-Teller distortions of D-2d symmetry of the CuBr42- complex. Some bands are split in the low-temperature spectra by the spinorbit interaction of the Br ligands. The results are compared with those available for other CuBr42- systems. We also investigate the dynamics of the inorganic CuBr42- units in the 9.5-300 K temperature range through the intensity of the charge-transfer bands. Analogously to the CdBr42- tetrahedra in the pure crystal, CuB42- experiences reorientational motions upon varying the temperature which are correlated with the temperature dependence of the monoclinic beta parameter below the Pmcn --> P2(1)/c phase transition temperature T-cl = 272 K. An important finding of the present work is the observation of anomalies in the CuBr42- dynamics which are associated with the existence of a new first-order phase transition at T-c2 = 20 K with a thermal hysteresis Delta T = 10 K. This new phase transition, which had not previously been detected in the [(CH3)(4)N](2)MBr(4) (M = Zn, Mn, Co or Cd) series, would correspond to the monoclinic P2(1)/c to orthorhombic P2(1)2(1)2(1) transition, confirming the predictions of the universal p-T phase diagrams of the title compounds.