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

The mechanisms behind the phase formation of Sr x Nb y O (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 SrNb 2 O 6 , Sr 2 Nb 2 O 7 and Sr 5 Nb 4 O 15 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 SrNb 2 O 6 , Sr 2 Nb 2 O 7 and Sr 5 Nb 4 O 15 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 Nb 2 O 5 to Nb 6 O 19 8− and finally to NbO 6 7− . These polyanions favor the formation of the different crystalline phases, as high concentration of Nb 6 O 19 8− the preferential formed phases are SrNb 2 O 6 , and Sr 2 Nb 2 O 7 , while high concentration of NbO 6 7− induced the formation of Sr 5 Nb 4 O 15 as a majority phase. The concentration of NaOH also plays a decisive role in the precipitation of SrNb 2 O 6 , Sr 2 Nb 2 O 7 and Sr 5 Nb 4 O 15 perovskites, high NaOH concentration promotes the Sr 2+ complexation favoring the formation of Sr 2+ rich phases.