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Ce(3+)- and Eu(3+)-doped YPO(4) nanorods have been prepared at relatively low temperature (120 degrees C). A detailed investigation of the role of Ce(3+) concentration up to 10 atom % on the luminescence intensity of Eu(3+) in Ce(3+)- and Eu(3+)-doped YPO(4) has been carried out. Phase transformation from a tetragonal to a hexagonal structure occurs with increasing Ce(3+) concentrations, and water molecules are also associated during phase transformation. Thermal study shows that water can be retained up to 800 degrees C in the hexagonal structure. Interestingly, the hexagonal structure returns to the tetragonal structure on annealing above 900 degrees C. As-prepared and 500 degrees C heated samples show uniform sized nanorods, whereas a 900 degrees C heated sample shows distorted nanorods in which pores are present. Initially, the luminescence intensity decreases sharply with increasing Ce(3+) concentrations, even for 2 atom %. This is related to the enhanced nonradiative rate as compared to the radiative rate, since multiphonon relaxation to surrounding water molecules increases. This is not due to the possible oxidation-reduction process between Eu(3+) and Ce(3+) to give Eu(2+) and Ce(4+), as confirmed by X-ray photoelectron spectroscopy and luminescence studies. Then, a significant enhancement of luminescence intensity occurs on annealing above 900 degrees C. This can be ascribed to the loss of water molecules during a phase transformation from the hydrated hexagonal to the dehydrated tetragonal phase. To the authors' knowledge, we for the first time performed a luminescent study with a change of solvent from H(2)O to D(2)O, and significant enhancement in luminescence is found.

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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.

When Eu(3+) ions occupy Ca(2+) sites of CaMoO(4), which has a body centered tetragonal structure with inversion symmetry, only the magnetic dipole transition ((5)D(0)→(7)F(1)) should be allowed according to Judd-Ofelt theory. Even if there are a few distortions in the Eu(3+) environment, its intensity should be more than that of the electric dipole transition ((5)D(0)→(7)F(2)). We report here the opposite effect experimentally and ascribe this to the polarizability effect of the MoO(4) tetrahedron, which is neighboring to EuO(8) (symmetric environment). The contribution of the energy transfer process from the Mo-O charge transfer band to Eu(3+) and the role of Eu(3+) over the surface of the particle could be distinguished when luminescence decay processes were measured at two different excitations (250 and 398 nm). Further, the luminescence intensities and lifetimes increase significantly with increasing heat-treatment temperature of the doped samples. This is attributed to the reduction of H(2)O from the surface of the particles and a non-radiative process after heat treatment.

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