Solvothermal Synthesis and Formation Mechanism of Potassium Sodium Niobate Mesocrystals Under Low Alkaline Conditions.

Pure-phase (K, Na)NbO3 (KNN) powders with orthorhombic symmetry were successfully synthesized by solvothermal method using isopropanol as solvent, without the addition of water. The as-prepared powders were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry to show the variation of phase, morphology, size distribution and chemical composition under different synthetic conditions, such as fill factors (FF) of the solvothermal system and alkalinity of the starting solution. Compared with the traditional hydrothermal method and the so-called solvothermal method (water aided in fact), small grains with well crystallinity were obtained using 100% isopropanol as reaction medium. The results indicate that both fill factor and alkalinity have significant effects on the phase structure and size distribution of the as-obtained KNN powders. Pure orthorhombic perovskite-structured KNN powders with a grain size of 100 nm were synthesized at the following condition: reaction time, 16 h; reaction temperature, 240 °C; fill factor, 70%; and alkalinity, 1 M. Small grains (100 nm) tend to form mesocrystals (10 µm) with tetrakaidecahedron structures, and the possible formation mechanism was proposed. The solvothermal method without the addition of water is a promising alternative to synthesize pure and refined powders under mild reaction conditions.