One-pot hydrothermal synthesis and formation mechanism of SrNb2O6–Sr2Nb2O7–Sr5Nb4O15 lamellar perovskites in highly concentrated NaOH solutions

Abstract

The mechanisms behind the phase formation of SrxNbyO(x+5 y/2) perovskites in hydrothermal reactions are yet not well understood. In this work, we report on the one-pot hydrothermal synthesis of SrNb2O6, Sr2Nb2O7 and Sr5Nb4O15 layered perovskites in highly concentrated NaOH solutions. The mechanisms that govern the crystallization and the crystallite morphology of the synthesized perovskites are explained as a function of the NaOH concentration. The crystal structure, morphology, and optical properties of the obtained SrNb2O6, Sr2Nb2O7 and Sr5Nb4O15 perovskites were analyzed using XRD, EDS, HR-TEM, UV–vis absorption, diffuse reflectance spectroscopy (DRS), and Raman spectroscopy. The element composition and resultant crystalline phases are determined by using EDX and Rietveld refinement techniques, respectively. NaOH concentration is the main parameter for the control over the phase formation. High concentration of NaOH accelerates the hydrolysis rate for the transformation from Nb2O5 to Nb6O198− and finally to NbO67−. These polyanions favor the formation of the different crystalline phases, as high concentration of Nb6O198− the preferential formed phases are SrNb2O6, and Sr2Nb2O7, while high concentration of NbO67− induced the formation of Sr5Nb4O15 as a majority phase. The concentration of NaOH also plays a decisive role in the precipitation of SrNb2O6, Sr2Nb2O7 and Sr5Nb4O15 perovskites, high NaOH concentration promotes the Sr2+ complexation favoring the formation of Sr2+ rich phases.