Recombination processes in rare-earth doped MAl2O4 (M=Ca,Sr) persistent phosphors investigated by optically-detected magnetic resonance

Single crystalline MAl2O4 (M=Ca and Sr) persistent phosphors, which are nominally pure or additionally doped with Eu and Nd or Dy, respectively, were investigated for their recombination luminescence (RL) and microwave-induced changes in the RL at low temperatures. The analysis of the optically-detected electron paramagnetic resonance (EPR) spectra, recorded after ultraviolet excitation at 4.2 K, shows that intrinsic donor and acceptor centres are involved in the recombination process. Spectral dependent RL-EPR measurements on undoped CaAl2O4 (CAO) and SrAl2O4 (SAO) show that we deal with only one donor but at least two different acceptors. The g value of the donor is 1.99 in CAO and 1.97 in SAO; the g value of the acceptors is about 2.02. The donor-acceptor recombination energy is either emitted directly (undoped samples) or almost completely transferred to the rare-earth activators (doped samples). (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)