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

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 center dot 4.5H(2)O with a plate-like morphology, a new phase of K2Nb2O6 center dot 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 degrees C, respectively. The reaction conditions are much milder than those in the normal hydrothermal process. Furthermore, the K2Nb2O6 center dot 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 degrees 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.