Charge-transfer spectra and dynamics of CuBr42- in (N(CH3)4)2CdBr4:Cu2+ crystals: a new first-order phase transition at Tc2 = 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 D2d symmetry of the CuBr42- complex. Some bands are split in the low-temperature spectra by the spin-orbit 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, CuBr42- experiences reorientational motions upon varying the temperature which are correlated with the temperature dependence of the monoclinic beta parameter below the Pmcn to P21c/ phase transition temperature Tcl = 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 Tc2 = 20 K with a thermal hysteresis Δ T = 10 K. This new phase transition, which had not previously been detected in the ((CH3)4N)2MBr4 (M=Zn, Mn, Co or Cd) series, would correspond to the monoclinic P21c/ to orthorhombic P212121 transition, confirming the predictions of the universal p-T phase diagrams of the title compounds. (author)