Photoluminescence and thermoluminescence in SBN:Cr crystals

Abstract

Cr doping in Strontium Barium Niobate (SBN) crystals is found to enhance the photorefractive response. Absorption bands around 650nm and below 600nm 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 765nm 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 88K and the other around 220K. 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±0.02eV. For the VIS-centers, the thermal activation energy estimated from their decay dynamics to be 0.36±0.05eV (Ming Gao, PhD thesis, University of Osnabrück, Germany, 1998), agrees with the activation energy value 0.30±0.05eV determined from the TL process.