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

This study investigates the hydrothermal synthesis of KNbO-NaNbO particles, which contains compositions of interest as lead-free piezoelectric materials. Precursor solutions containing NbO, 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 NaNbO, whereas solely KOH resulted in hexagonal plate-like KNbO. When 75 % KOH was used, the product was a mixture of large KNaNbO·9 HO hexagonal plates and smaller NaNbO 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 KNaNbO·HO hexagonal plates, which could be transformed to KNaNbO 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 NbO and KOH/NaOH.