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Yttrium niobate YNbO4 nanocrystals with ellipsoidal morphology were directly formed from the precursor solution mixtures of YCl3 and NbCl5 under weakly basic conditions in the presence of aqueous ammonia by hydrothermal method. The hydrothermal treatment at 180°C for 5 h was necessary to obtain nanocrystals (18 nm) with sufficient crystallinity. The optical band gap of the as-prepared samples was 3.6 eV. The as-prepared YNbO4 nanocrystals showed UV-blue and broadband emission centered at 405 nm under excitation at 235 nm, which was due to the blue recombination luminescence, associated with charge-transfer transitions involving the tetrahedral NbO4 group. The emission intensity increased with increased hydrothermal treatment temperature. The photoluminescence intensity of the YNbO4 was extremely improved via heating above 1000°C in air, which was accompanied by the increase in the optical band gap from 3.6 to 4.0 eV. By heat treatment at 1300°C, the intensity of the UV-blue and broadband emission (with maximum at 400 nm) for the YNbO4 became more than 22.5 times as strong as that before heat treatment.