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 °C 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 K_0.7Na_0.3NbO₃ 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.