Luminescent nanocrystals having distinctive ellipsoidal morphology and the complete solid solution of monoclinic phase in the YNbO4-EuNbO4 system were hydrothermally formed under weakly basic conditions at 180-240 degrees C for 5 h. The excitation spectra of the Y1-xEuxNbO4 nanocrystals, which consisted of the charge transfer (CT) band around 240-270 nm due to [NbO4](3)-Eu3+ interaction and several sharp bands corresponding to the f-f transitions of Eu3', e.g. the most intense absorption band at 395 nm (F-7(0)-L-5(6)), showed that the compounds could be excited by both ultraviolet and visible light. A dominant red emission at 610 nm and a weak orange emission at 590 nm assigned to D-5(0)-F-7(2) and D-5(0)-F-7(1) transitions of Eu3+, respectively were observed in the as-prepared nanocrystals containing europium under excitation at 395 nm. The heat treatment at temperatures more than 1000 'C in air was effective for the improvement in the photoluminescence intensity of the as-prepared solid solutions. By the enhancement in the crystallinity and crystallite growth of the monoclinic phase through heat treatment at 1300 C in air, the luminescence intensity of the as-prepared solid solution, Y0.75Eu0.25NbO4 (x = 0.25), which showed the most significant photoluminescence, became more than 5 times as strong as that before the heat treatment. (C) 2013 Elsevier Ltd. All rights reserved.