Abstract
Ce3+ and Sm3+ both form stable defect centres in Y PO4, and their emission properties are well known. However, by irradiating co-doped Y PO4:Ce,Sm with x-rays or UV light the charge states of the defects can be modified to become Ce4+ and Sm2+, which are metastable, and their behaviour acts as a model system for understanding carrier dynamics in charge storage phosphors. Here we report on the luminescence emission behaviour of the Sm2+ defects that can be observed after x-irradiation. Under suitable excitation conditions, emission from both the stable Sm3+ and metastable Sm2+ can be monitored simultaneously. The Sm2+ luminescence is found to be comprised of a series of narrow lines in the energy range 1.5–1.8 eV, identifiable as internal 4f–4f transitions, accompanied by a series of phonon replicas (phonon energy, 20.4 meV). The intensity of the metastable Sm2+ emission increases in proportion to the x-irradiation time as their population is increased; under 1.92 eV laser stimulation, the PL is found to fully quench at temperatures above 150 K due to photo-thermal ionization of the defect.
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