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

Kenji Kaseda, Masafumi Takesue, Taku M. Aida, Masaru Watanabe, Hiromichi Hayashi, Richard L. Smith

Research output: Contribution to journalArticlepeer-review

10 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)279-285
Number of pages7
JournalJournal of Supercritical Fluids
Issue number2
Publication statusPublished - 2011 Sep 1


  • Defect pyrochlore
  • Hydrothermal
  • Pb-free piezoelectric
  • Perovskite
  • Potassium niobate

ASJC Scopus subject areas

  • Chemical Engineering(all)
  • Condensed Matter Physics
  • Physical and Theoretical Chemistry


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