Thermoluminescence and defect centres in Tb doped lithium magnesium borate phosphor

Abstract

Terbium doped lithium magnesium borate phosphor exhibits thermoluminescence (TL) peaks at about 140°C, 200°C, 225°C and 370°C. The phosphor was characterized by X-ray powder diffraction and photoluminescence studies. Electron Spin Resonance (ESR) studies were carried out to identify the defect centres responsible for the TL peaks. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0108 is identified as an O− ion and the centre correlates with the TL peak at 200°C. Centre II with an isotropic g-factor 2.0029 is assigned to an F+-type centre (singly ionized oxygen vacancy) and is the likely recombination centre for the TL peaks at 200°C and 225°C. An additional defect centre is observed during thermal annealing experiments and this centre (assigned to F+ centre) seems to originate from an F centre (oxygen vacancy with two electrons). This F centre may be related to the observed high temperature 370°C TL peak in LiMgBO3:Tb phosphor.