Evidence of thermal ionization induced luminescence quenching in CePO4 and GdPO4

Abstract

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.