Synthesis of strontium bismuth niobate (SrBi 2Nb 2O 9) using an aqueous acetate–citrate precursor gel: thermal decomposition and phase formation

Abstract

An aqueous solution–gel route was developed for the preparation of the ferroelectric material strontium bismuth niobate (SrBi 2Nb 2O 9, SBN). Starting from aqueous Bi 3+-acetate and Sr 2+-acetate solutions and an aqueous peroxo-citrato-Nb 5+-precursor solution, this method offers a low-cost and environmentally friendly alternative to the conventional sol–gel techniques. With regard to the deposition of thin films, it is important to gain insight in the behaviour of the precursor during thermal treatment. For this reason the thermal decomposition mechanism of the precursor gel was studied: weight loss and evolved gases were characterized by (hyphenated) TGA–MS and TGA–FTIR, while changes in the solid state during heating were detected using high temperature diffuse reflectance infrared fourier transform spectroscopy (HT-DRIFT). The decomposition in dry air can be divided in four temperature regions. The first two steps are ascribed to the drying of the sample and to the decomposition of the organic matrix, not coordinated to the metal ions. During the third step the coordination sphere of the metal ions is decomposed and finally the residual organic matter is combusted in the fourth region. TEM experiments on freestanding thin films showed that the precursor homogeneity was maintained throughout the entire thermo-oxidative decomposition of the gel. The phase formation of SBN was studied by means of high temperature X-ray diffraction (HT-XRD). At about 425 °C the intermediate fluorite phase is crystallized out of the amorphous precursor. The phase transformation into the desired ferroelectric perovskite phase seems to be dependent on the Bi-content: an excess of Bi lowers the onset temperature of perovskite formation.