Nanopowders of YPO4 phosphors with different Pr3+ doping were successfully prepared by a sol gel method under different synthesis conditions. The crystallite size and strain show a strong dependence on the Pr3+ doping concentration and on the annealing temperature. By annealing at 300 degrees C one can obtain the xenotime structure of the pure YPO4. The crystallite size can be controlled by controlling the annealing temperature and it increases with increasing the annealing temperature. The room temperature inter-configurational 4f(2) <-> 4f5d and intra-configurational 4f(2) <-> 4f(2) emission-excitation transitions spectra are measured and investigated. Upon 4f(2) <- 4f5d excitation transition, all the samples present broad intense emission bands attributed to 4f5d -> 4f(2) emission transitions and peaks in red region assigned to D-1(2) -> H-3(4) transition as photon cascade emission phenomena (PCE). The presence of only D-1(2) -> H-3(4) transition is discussed. In addition, the D-1(2) energy level lifetimes as well as the refractive indexes were determined and discussed. (c) 2018 Elsevier B. V. All rights reserved.