Revealing the hydrothermal crystallization mechanism of ilmenite-type sodium niobate microplates: the roles of potassium ions

Abstract

In this study, ilmenite-type NaNbO₃ microplates were prepared by a facile surfactant-free method, using NaAc and KOH as a sodium resource and as a mineralizer, respectively. The effects of each reactant were identified, and the formation process and crystallization pathway of ilmenite-type NaNbO₃ microplates were explored based on the phase constitution, microstructure and chemical compositional analysis. The indispensable roles of K ions, which participated in the formation of layered KNN-hydrate intermediates, were revealed. Ilmenite-type NaNbO₃ microplates were formed by the dissolution of these intermediates and subsequent recrystallization into K-containing NaNbO₃, followed by the ion-exchange between K and Na ions. Meanwhile excess K ions would hinder the release of K ions associated with the dissolution of intermediates, thus preventing the formation of ilmenite-type NaNbO₃. Moreover, the morphological evolution of ilmenite-type NaNbO₃ microplates from platelets to octahedra was interpreted in terms of their intrinsic structure and KOH concentration.