FORMATION OF LEAD NIOBATES IN MOLTEN-SALT SYSTEMS

The formation mechanisms, phase stability, impurity incorporation, and morphological characteristics of lead niobates obtained by molten salt synthesis were studied by X-ray diffraction analysis, scanning electron microscopy, and energy dispersive analysis. Results indicate that PbO is the fast-dissolving component of the constituent oxides and that the formation of lead niobates initiates at the surface of Nb2O5 powders. Reaction rates were faster in molten salts when compared to those of solid-state synthesis. Significant morphological differences were observed between the powders prepared by both solid-state and molten salt synthesis. At low PbO/Nb2O5 molar ratios, reaction between starting powders and molten salts resulted in the formation of NaNbO3, along with potassium incorporation into lead niobates. Orthorhombic PbNb2O6, monoclinic Pb2Nb2O7, and fcc Pb3Nb3O8 phases were stabilized using molten salt synthesis, while only rhombohedral PbNb2O6 and Pb2Nb2O7, and tetragonal Pb3Nb2O8, were obtained by solid-state synthesis. Molten salt compositions containing PbO/Nb2O5 ratios of 2 and 3 resulted in the stabilization of phases different from those obtained using solid-state synthesis. For a PbO/Nb2O5 ratio equal to 3, Pb5Nb4O13 observed in solid-state synthesis was not found in the products of molten salt synthesis.