Effect of stirring and KOH/NaOH ratio on phase formation in hydrothermal synthesis of KNbO3-NaNbO3 particles

This study investigates the hydrothermal synthesis of KNbO3-NaNbO3 particles, which contains compositions of interest as lead-free piezoelectric materials. Precursor solutions containing Nb2O5, NaOH/KOH were hydrothermally reacted at 190 0C for 15 hours using either a conventional reactor or a reactor with stirring. For 50 % KOH or lower, the product was cubic or elongated cuboid/belt-shaped NaNbO3, whereas solely KOH resulted in hexagonal plate-like KNbO3. When 75 % KOH was used, the product was a mixture of large K5Na3Nb6O19·9 H2O hexagonal plates and smaller NaNbO3 cubes. We hypothesized this phase segregation was induced by highly concentrated microenvironments, and stirring would prevent that. Accordingly, stirring hydrothermal treatment of 75 % KOH solution produced only K5Na3Nb6O19·H2O hexagonal plates, which could be transformed to K0.7Na0.3NbO3 by calcination. By reducing the hydrothermal treatment time to 1 – 6 hours we isolated reaction intermediates, and based on them propose a kinetics-controlled mechanism for hydrothermal reactions of Nb2O5 and KOH/NaOH.