Monoclinic fergusonite-type niobate, YNbO4:Tb3+ and complete solid solutions with luminescent properties in the YNbO4-TbNbO4 system were directly formed as nanocrystals from the precursor solutions of NbCl5, TbCl3, and YCl3 under hydrothermal conditions at 240 degrees C. The niobate nanocrystals possessed distinguishing feature of ellipsoidal morphology. The optical band gap of the as-prepared samples, Y1-xTbxNbO4, x = 0-1.0, decreased from 3.7 to 2.9 eV as the terbium concentration increased from x = 0 to 1.0. The photoluminescence spectra of the as-prepared nanocrystals containing terbium are responsible for the characteristic blue (490 nm) and green (545 nm) luminescence, associated with D-5(4) -> F-7(6) and D-5(4) -> F-7(5) transitions, respectively. The strongest emission bands in the green spectral domain corresponding to D-5(4) -> F-7(5) transition of Tb3+ are located at 545 and 550 nm under excitation at 240 nm Xe lamp. By heat treatment at 1300 degrees C, the luminescence intensity of the as-prepared solid solution x = 0.10 that presented the highest photoluminescence intensity increased more than 8 times as high as that before heat treatment. (C) 2013 Elsevier B.V. All rights reserved.