Solid Solution Nanocrystals in the CeO2‐Y3NbO7 System: Hydrothermal Formation and Control of Crystallite Growth of Ceria

New solid solution nanocrystals with fluorite‐type cubic structure in the ceria (CeO₂)‐yttrium niobate (1/4Y₃NbO₇) system were directly formed at 120°C–240°C from the precursor solution mixtures of (NH₄)Ce(NO₃)₆, YCl₃·6H₂O, and NbCl₅ under mild hydrothermal conditions in the presence of aqueous ammonia. The hydrothermal formation of cubic solid solution nanocrystals in the wide composition range of CeO₂ (mol%) = 10–100 in the CeO₂–1/4Y₃NbO₇ system was effectively achieved via the assistance of the presence of CeO₂ component more than 10 mol% as a promoter with the same fluorite‐type structure. The optical band gap of the solid solutions gradually decreased with increased CeO₂ component. The high phase stability of the solid solutions in the CeO₂–1/4Y₃NbO₇ system was confirmed, i.e., the single cubic phase of the solid solutions was maintained after heat treatment at 600°C–1500°C for 1 h in air. The presence of Y₃NbO₇ as an inhibitor and the substitutional incorporation of Y₃NbO₇ into the lattice, CeO₂ effectively controlled the crystallite growth of CeO₂, and nano‐sized cubic solid solutions with high specific surface areas were maintained after heat treatment up to 800°C–1000°C for 1 h air.