Evidence of thermal ionization induced luminescence quenching in CePO4 and GdPO4

The optical properties and their temperature dependence of two rare earth orthophosphates, CePO4 and GdPO4, are presented. CePO4 shows prominent luminescence emission at wavelengths of 392 and 425 nm, typical of interconfigurational 5d -> 4f transitions. For GdPO4, the prominent emission is observed at a wavelength of 313 nm, due to intraconfigurational 4f -> 4f transitions. Upon increasing the measurement temperature, the activation energy required to quench the luminescence emission is found to be 0.25 and 0.58 eV for CePO4 and GdPO4 samples, respectively. GdPO4 showed better thermal stability of the luminescence when compared to the CePO4 sample. Vacuum referred binding energy (VRBE) diagrams were prepared to determine the thermal quenching mechanisms in the two cases. It is observed that the quenching mechanism in both cases is dominated by thermal ionization of charges to the conduction band (CePO4) and the valence band (GdPO4). Thermoluminescence studies were performed to correlate the location of traps in the two samples and the corresponding thermal quenching mechanisms.