Synthesis of alkali niobate K_1-xNa_xNbO₃ nanoparticles using a supercritical water flow system

Alkali niobate, K_1-xNa_xNbO₃, nanoparticles (ca. 50-60 nm) were continuously synthesized from Nb-sol and NaOH/KOH solutions with a supercritical water flow system. The syntheses were carried out at temperatures from 400 to 480 °C, at pressures from 25 to 30 MPa, and at reaction times from 0.1 to 8.3 s. X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy-energy dispersion spectroscopy (SEM-EDS) techniques were used to characterize the samples. Single phase KNbO₃ was obtained at 480 °C and 25 MPa at an initial K/Nb molar ratio of 15. The K/Nb molar ratio of the products increased with an increase in the reaction time. For NaNbO₃, single phase product was formed under supercritical water conditions irrespective of the temperature and pressure. The K_1-xNa_xNbO₃ nanoparticles were synthesized at 480 °C and 25 MPa for varying Na/K concentrations. Single phase K_0.5Na_0.5NbO₃ was obtained at an initial Na/K molar ratio of 0.25. To form the perovskite structure, the rearrangement of NbO₆ units with alkaline ions is required. The rearrangement proceeds more rapidly with Na⁺ ions than with K⁺ ions probably due to the smaller ionic radius of Na⁺. Therefore, formation of K_0.5Na_0.5NbO₃ was achieved at the excess amount of K⁺ ions than Na⁺ ions.