Emission decay kinetics in a CaWO4:Bi crystal

The decay kinetics of the 2.88 eV emission of pure and Bi-doped CaWO4 crystals has been studied in the temperature region of 4.2-300 K. The decay components connected with the radiative transitions from the triplet excited states of regular WO42− oxyanionic complexes and WO42− complexes perturbed by Bi3+ ions (WO42−(Bi3+) centres) have been identified. It is shown that WO4(Bi3+) centres can be excited directly above the edge of the fundamental absorption of CaWO4, or due to the energy transfer from the metastable state of a Bi3+ ion at the excitation in the Bi3+ absorption band at 4.4 eV. At temperatures T>170 K, the excited state of the WO42−(Bi3+) centre is thermally ionised, giving rise to phosphorescence in the blue spectral region. These processes are compared with those known for excitons in PbWO4 crystals.