The optical properties of nanocrystalline YPO4:Ln3+ (Ln = Eu, Sm, Tb) prepared via co-precipitation are compared to larger crystallites of YPO4:Ln3+ prepared via traditional solid state reaction. In larger crystals (~330 nm) a distinct peak is observed at 150 nm in the excitation spectra, the intensity of which decreases markedly in smaller crystals (~20 nm). Using excitation and reflectance spectroscopy, host–to–activator energy transfer efficiencies were calculated for Y1-xPO4:Lnx3+ (0.01 ≤ x ≤ 0.10). From the transfer efficiency data, we estimate that trapping by Eu3+ and Sm3+ is at least five times more efficient than trapping by Tb3+ for excitation at the band edge. The fraction of energy lost to the surface or grain boundaries for excitation at 150 nm and 138 nm is also estimated. We propose that in the samples prepared via co-precipitation, an amorphous phase forms at grain boundaries that is responsible for the loss of efficiency under 150 nm excitation
