Studies of defects and optical properties of CaAl12O19:Ho3+ phosphor materials

Abstract

Phosphor CaAl12O19 doped with holmium was synthesized by the low temperature solution combustion method. Powder X-ray diffraction (XRD) technique was used to find the phase formation and phase purity. The optical properties were studied using diffuse reflectance and photoluminescence (PL) spectroscopy. CaAl12O19:Ho exhibits three thermoluminescence (TL) peaks at approximately 140 oC, 260 oC and 440 oC. Electron Spin Resonance (ESR) studies were carried out to study the defect centers induced in the phosphor by gamma irradiation and also to identify the centers responsible for the TL process. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of at least four distinct centers. One of the centers (center I) with an isotropic g factor 2.0115 is tentatively assigned to an intrinsic type center. This center relates with the TL peak at 140 oC. Center II with an axially symmetric g tensor with principal values =2.0283 and =2.0058 is identified as an ion and appears to correlate also with the 140 oC TL peak. Center III with an isotropic g-factor 2.0011 is assigned to an F+ -type center (singly ionized oxygen vacancy) and is the likely recombination center for the two low temperature TL peaks. Another center with an isotropic g-value 2.0043 is visible after 300 oC thermal anneal and is also attributed to an F+ center (center IV). This center appears to be associated with the high temperature TL peak at 440 oC.