Photoluminescence and thermoluminescence in SBN:Cr crystals

Cr doping in Strontium Barium Niobate (SBN) crystals is found to enhance the photorefractive response. Absorption bands around 650 nm and below 600 nm can be attributed to Cr in SBN crystals. In addition to XPS experimental results, the appearance of broad R line absorptions at low temperature confirms that most Cr ions are in the Cr3+ charge state in the SBN crystals. Luminescence bands around 765 nm were observed with different peak positions in nominally pure and in Cr-doped SBN crystals. The positions of the maxima in the excitation spectra in SBN:Cr, coinciding with the absorption bands, indicate that. this emission band comes from the excitation of Cr3+ ions. Two thermoluminescence (TL) peaks are found in SBN:Cr and nominally pure SBN crystals: one at 88 K and the other around 220 K. The identical spectral distribution of the two TL bands points to the same recombination process following the liberation of two light-induced electron trapping centers: Nb4+ polarons and VIS-centers created at low temperatures. A model of the thermal recovery process involved is proposed. The thermal activation energy for the hopping motion of Nb4+ polarons is estimated to be 0.18 +/- 0.02 eV. For the VIS-centers, the thermal activation energy estimated from their decay dynamics to be 0.36 +/- 0.05 eV (Ming Gao, PhD thesis, University of Osnabruck, Germany, 1998), agrees with the activation energy value 0.30 +/- 0.05 eV determined from the TL process. (C) 2000 Elsevier Science Ltd. All rights reserved.