LUMINESCENCE PROPERTIES OF CU-2+-DOPED TMA2MNBR4 CRYSTALS AND A SPECTROSCOPIC STUDY OF THE CUBR42- COMPLEXES FORMED

This work investigates the influence of Cu2+ impurities on the luminescence properties Of TMA2MnBr4:Cu2+ as well as the local structure and orientation of the CuBr42- complexes formed, by means of the excitation and luminescence spectra, lifetime measurements and polarized optical absorption spectroscopy in the 10-300 K temperature range. It is demonstrated that the presence of an intense Br --> Cu2+ charge transfer (CT) band at 555 nm strongly favours a direct energy transfer from Mn2+ to the non-luminescent Cu2+ impurities The influence of this energy transfer on the Mn2+ luminescence intensity, lifetime and bandshape is analysed as a function of the Cu2+ concentration. The results are compared with previous ones obtained in one-dimensional Cu2+-doped TMAMnCl3 and TMAMnBr3 crystals. Two x, y-polarized bands at 18 000 and 28 400 cm-1, and two z-polarized bands at 23 800 and 36 100 cm-1 are observed in the CT spectra Of TmA2MnBr4:Cu2+. Their transition energies as well as their polarization are explained in terms of D2d symmetry distortions of the CuBr42- tetrahedra. We also analyse the triplet structure observed in the first CT band which is associated with the tetrahedral 2T1 CT state, which is split by the effect of both the static D2d distortion and the large spin-orbit coupling of the Br- ligands. The absence of discontinuities in the evolution of the CuBr42- CT bands with temperature supports the finding that no structural phase transition occurs below 270 K in these crystals.