Direct synthesis of nanocrystalline GdNbO₄ and GdNbO₄-based phosphors doped with Eu³⁺ through hydrothermal route

Nanocrystalline gadolinium niobate, GdNbO4 with fergusonite structure was directly formed from the precursor solution mixtures of GdCl3 and NbCl5 under weakly basic conditions at temperatures higher than 210°C for 5 h in the presence of aqueous ammonia via hydrothermal route. The as-prepared GdNbO4 nanocrystals and those after heating at 1000–1300°C showed UV-blue and broad-band emission centered at 435 nm under excitation at 266 nm due to the blue recombination luminescence, associated with charge transfer transitions involving the tetrahedral NbO4 group. The fergusonite-type complete solid solution nanocrystals having good luminescence in the red spectral region in the GdNbO4–EuNbO4 system were directly formed under hydrothermal conditions at 240°C. The photoluminescence spectra of the as-prepared nanocrystalline solid solutions containing europium under excitation at 240 nm are responsible for the characteristic weak orange luminescence (592 nm) and strong red luminescence (611 nm), associated with 5D0 → 7F1 and 5D0 → 7F2 transitions, respectively. In the composition Gd0.80Eu0.20NbO4, the maximum intensity of red emission was observed and the emission was even more enhanced via heating at 1300°C.