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Cubic solid solution nanocrystals in the zirconia (ZrO2)–yttrium niobate (Y3NbO7) system were directly formed at 180°C–240°C from the precursor solution mixtures of NbCl5, ZrOCl2, and YCl3 under mild hydrothermal conditions in the presence of aqueous ammonia. The lattice parameter corresponding to cubic phase linearly changed according to the Vegard's law in the wide composition range of ZrO2 (mol%) = 10–90 in the ZrO2–1/4Y3NbO7 system. The progress of the formation of nanocrystalline solid solutions based on the Y3NbO7 was assisted via the presence of ZrO2 component as a promoter with the same fluorite-type structure. The optical band gap of the solid solutions was in the range 3.4–3.7 eV. Broadband emissions centered at 360–380 and 390 nm were observed for the nanocrystalline cubic solid solutions and pure cubic Y3NbO7, respectively, under excitation at 240 nm Xe lamp. The nanosized cubic crystallites of the solid solutions were maintained after heat treatment up to 800°C for 1 h air. The cubic phase of the solid solutions in the ZrO2–1/4Y3NbO7 system was maintained after heat treatment at 1400°C in air.