Synthesis And Luminescence Properties Of Gd, Eu Co-doped LaPO₄

Due to the unique 4f electronic shell, lanthanide elements show outstanding optical properties, which make them have bright applications prospect in nanophosphors field. Establishing new synthetic strategies of these materials to make the atomic arrangement controllable in the nano-scale; understanding the relations between morphology and luminescent properties and raising the stability and intensity of materials practically are crucial to deep processing and detailed study of rare earth phosphors.In this article, chemical precipitation method and hydrothermal method are used to synthesize LaPO₄ :Eu³⁺,Gd³⁺, with Eu2O₃,La(NO₃)3·6H₂O,Gd(NO₃)3 and H3PO₄ as raw materials and ammonia as pH regulator. The phase, morphology and luminescent properties of products are investigated by XRD,DTA,SEM and spectrofluorometers; the effect of pH values, thermal treatment temperature, doped concentration of ions and surfactant on the luminescent properties are studied.The experimental results show that the products are fasciculated hexagonal LaPO₄ synthesized by chemical precipitation method under acidic condition at room temperature, which have a length of 300-700nm; the phase is still hexagonal with a decreased length-diameter ratio under basic condition but the morphology is branch-like; when pH=7, the grain-like cluster products have no obvious diffraction peaks. Photoluminescence analysis shows that the products have the greatest luminescent properties when pH=7, and the transition intensities of (5D0â†'7F1) and (5D0â†'7F2) are equal, indicating a non-centro-symmetrical environment for Eu³⁺ with orange-red emission. The phase,morphology and luminescent properties are changeable after the thermal treatment. Monoclinic powders replace hexagonal ones gradually, and products have a totally monoclinic phase and greatest intensity after calcinations at 1000℃. Meanwhile powders gather into large massive particles. The best concentration of doped ions is confirmed by contrasting photoluminescence intensity of powders with different ion concentrations, the best values of Gd³⁺ and Eu³⁺ are 2.5%mol and 5%mol respectively. Concentration quenching occurs with increasing concentration. The energy transfer mode between Gd³⁺ and Eu³⁺ is resonance transfer dominated by electric multipole interaction.In the second part, LaPO₄ :Eu³⁺,Gd³⁺ phosphors(pH=9) are synthesized by hydrothermal method at different temperatures. The obtained LaPO₄ is hexagonal phase and inhomogenous linear structure after 100℃. When the temperature is 200℃, the phase turns to monoclinic, and the morphology turns to smooth columnar. Compared with ones calcined at 1000℃, products synthesized by hydrothermal method have a low degree of crystallization and a low luminescence intensity, which caused by low temperature, bad sealed autoclave, or not long time. Products have a growth trend from linear ones to particles with the addition of PEG2000. This is because Steric Hindrance Effect of macromolecule suppresses the anisotropy growth."Growth unit"theory is used to explain the growth of crystal under hydrothermal condition. The"unit"control the rate of reaction ions by aggregating on some interfaces to determine the final morphology of crystals.