Transformation of potassium Lindquist hexaniobate to various potassium niobates: solvothermal synthesis and structural evolution mechanism

Abstract

This paper introduces the formation reactions and reaction mechanisms of a series of potassium niobates from a potassium salt of the Lindquist hexaniobate [Nb₆O₁₉]⁸⁻ ion under solvothermal conditions. The structure and particle morphology of the potassium niobate product can be controlled easily with the reaction solution alkalinity using this solvothermal process. KNb₃O₈ with a plate-like morphology, K₄Nb₆O₁₇·4.5H₂O with a plate-like morphology, a new phase of K₂Nb₂O₆·H₂O with fibrous morphology, KNbO₃ perovskites with cubic morphology are obtained at pH = 5.5, and in 0.3, 0.5, 1.0 mol L⁻¹ KOH solutions at 230 °C, respectively. The reaction conditions are much milder than those in the normal hydrothermal process. Furthermore, the K₂Nb₂O₆·H₂O fibers can be topotactically transformed into KNbO₃ fibers, Nb₂O₅ fibers after H⁺-exchange-treatment, and LiNbO₃ fibers after Li⁺-exchange-treatment by heat-treatments at 730, 560, and 520 °C, respectively. The formation reaction and structure of these potassium niobates were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), energy-dispersive spectroscopy (EDS), Raman spectra and TG-DTA. The formation mechanism of this series of potassium niobates from the [Nb₆O₁₉]⁸⁻ precursor is systematically explained via the correlation between the octahedrons [NbO₆] sharing forms in the precursor structure and in the product structures.