Synthesis of anisometric KNbO3 and K0.5Na0.5NbO3 single crystals by chemical conversion of non-perovskite templates

Abstract

KNbO₃ (KN) and K_1−xNa_xNbO₃ (KNN) anisometric crystals have been produced by a chemical conversion route. Crystals of the compounds K₄Nb₆O₁₇, K₂Nb₄O₁₁ and KNb₃O₈ with high aspect ratio were first grown in molten KCl. The non-perovskite compounds were further converted to KN or KNN perovskites by reaction with K₂CO₃ or Na₂CO₃ at 800 °C with or without the presence of molten KCl. Through a dissolution–precipitation mechanism needle- or plate-like KNN and KN particles with high aspect ratios were formed with preferred orientation along the [011] or [100] directions. A short reaction time was observed to yield complete chemical conversion to polycrystalline KN/KNN, while single crystals with the morphology resembling the non-perovskite templates were obtained after longer reaction time by Ostwald ripening.