EPR, ENDOR and HYSCORE study of X-ray induced centres in K2YF5 thermoluminescent phosphors

Abstract

X-Ray irradiation at room temperature produces several paramagnetic centres in rare-earth activated K₂YF₅ crystals, whose thermal annealing behaviour can be linked with the occurrence of thermoluminescence (TL) glow peaks. In this paper, continuous wave (CW) and pulsed paramagnetic resonance techniques are used to study the structure of a very stable radiation-induced centre, which may be involved in the TL peak at ∼390 °C reported for Ce- and Tb-activated crystals. From the spectra the centre’s g tensor and hyperfine (nuclear quadrupole) tensors for several ¹⁹F and ³⁹K neighbouring nuclei are extracted, but no self-hyperfine interaction could be detected. Based on the analysis of the interaction tensors, a model is constructed consisting of an oxygen-related radical (e.g.O⁻ or O₂⁻) on a substitutional F⁻ position in the mirror plane of the YF₇ polyhedra. Such a centre most probably corresponds to a trapped-hole state.