Disappearance and recovery of luminescence in Bi3+, Eu3+ codoped YPO4 nanoparticles due to the presence of water molecules up to 800 °C.

YPO(4) nanoparticles codoped with Eu(3+) (5 at. %) and Bi(3+) (2-10 at. %) have been synthesized by a simple coprecipitation method using a polyethylene glycol-glycerol mixture, which acts as capping agent. It has been found that the incorporation of Bi(3+) ions into the YPO(4):Eu(3+) lattice induces a phase transformation from tetragonal to hexagonal, and also a significant decrease in Eu(3+) luminescence intensity was observed. This is related to the association of the water molecules in the hexagonal phase of YPO(4) in which the nonradiative process from the surrounding water molecules around Eu(3+) is dominating over the radiative process. On annealing above 800 °C, luminescence intensity recovers due to significant removal of water. 900 °C annealed Bi(3+) codoped YPO(4):Eu(3+) shows enhanced luminescence (2-3 times) as compared to that of YPO(4):Eu(3+). When sample was prepared in D(2)O (instead of H(2)O), 4-fold enhancement in luminescence was observed, suggesting the extent of reduction of multiphonon relaxation in D(2)O. This study illustrates the stability of water molecules even at a very high temperature up to 800 °C in Eu(3+) and Bi(3+) codoped YPO(4) nanoparticles.