Reaction mechanism for the solid state synthesis of LaPO4:Ce, Tb phosphor

LaPO4:Ce, Tb is an efficient green component of trichromatic lamp phosphors. It is usually prepared by solid state reaction of a blend of (NH4)2HPO4, Tb4O7 and CeO2 with or without Li2CO3 flux. This reaction mechanism has been investigated by differential thermal analysis, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy. Experimental results show that at approximately 200 ??C (NH4)zHPO4 decomposes to form syrupy P2O5&middotxH2O. As the firing temperature is increased to 600 ??C (in the presence of Li2CO3) and 800 ??C (in the absence of Li2CO3) syrupy P2O5 begins to react with rare earth oxides to form mixed rare earth orthophosphates. Up to 1000 ??C and 1400 ??C, respectively, these two reactions go to completion. Their reaction rates change linearly as the reciprocal temperature and with different slopes, from which activation energies are estimated to be 22 and 35 kcal mol-1, respectively. Reactions appear to be diffusion-limited. Thermal quenching of luminescence of LaPO4:Ce,Tb can be minimized by the addition of Li2CO3 in the reaction mixture.EI0406-408207-