This paper provides the detailed study of (nano)particle's size effect on structural and luminescent properties of LaPO 4 :Eu 3+ synthesized by four different methods: high temperature solid-state, co-precipitation, reverse micelle and colloidal. These methods delivered monoclinic monazite-phase submicron particles (> 100 nm), 4 × 20 nm nanorods and 5 nm spheres (depending on the annealing temperature), 2 × 15 nm nanorods, and ultrasmall spheres (2 nm), respectively. The analysis of emission intensity dependence on Eu 3+ concentration showed that quenching concentration increases with a decrease of the particle size. The critical distance for energy transfer between Eu 3+ ions is found to be 18.2 Å, and the dipole-dipole interaction is the dominant mechanism responsible for the concentration quenching of emission. With the increase in Eu 3+ concentration, the unit-cell parameter slightly increases to accommodate larger Eu 3+ ions at sites of smaller La 3+ ions. Photoluminescent emission spectra presented four characteristic bands in the red spectral region: at 592 nm (5 D 0 → 7 F 1), at 612 nm (5 D 0 → 7 F 2), at 652 nm (5 D 0 → 7 F 3) and at 684 nm (5 D 0 → 7 F 4), while in small colloidal nanoparticles additional emission bands from host defects appear at shorter wavelengths. Intensities of f-f electronic transitions change with particles size due to small changes in symmetry around europium sites, while emission bandwidths increase with the reduction of particle size due to increased structural disorder. Judd-Ofelt analysis showed that internal quantum yield of Eu 3+ emission is strongly influenced by particle's morphology.

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Preparation and spectroscopic studies of the TiO 2 nanopowders doped with Eu 3+ ions are described. Efficient emission in the red part of the visible spectrum can be obtained due to theF 2 emission of europium ions. Quantum efficiency of such emission was estimated to be about 0.83, which indicates a rather weak role of the non-radiative losses. However, the increase of Eu 3+ concentration up to 10 at% significantly lowers the quantum efficiency because of the energy transfer and re-absorption processes. Higher doping concentrations (larger than 3 at% of Eu 3+ ) also decrease the covalency of the Eu 3+ O 2¹ chemical bonds.

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