Restructuring mechanism of NbO₆ octahedrons in the crystallization of KNbO₃ in supercritical water

Fine particles of KNbO₃ can be synthesized on a time scale of seconds in supercritical water with a flow-type reactor. The restructuring of NbO₆ octahedrons was found to be a primary factor in the mechanism of KNbO₃ crystallization in high-temperature aqueous environments. Two different crystallization routes exist depending on the conditions of water. The ion product of water affects whether the restructuring of edge-sharing NbO ₆ octahedrons to corner-sharing NbO₆ octahedrons proceeds directly or stepwise according to the degree of dehydration. The direct crystallization route provides polygonal-shaped KNbO₃ particles via direct restructuring, namely, homogenous nucleation from supercritical water that is strongly correlated with a low ion product. The stepwise crystallization route provides irregularly shaped KNbO₃ particles via restructuring in stages to produce an intermediate K₄Nb₆O₁₇ phase that is correlated with a high ion product. As the ion product of water decreases in high-temperature water, the stability of crystalline phase is in order of KNbO₃ > K₄Nb₆O₁₇ > K₄Nb₆O₁₇·3H₂O > defect pyrochlore due to the suppression of dehydration. Water acts to not only dominate the restructuring mechanism of NbO₆ octahedrons but also to provide efficient transport of K⁺ and OH^- ions that allows rapid restructuring of NbO₆ octahedrons during KNbO₃ crystallization.