Synthesis of oxide perovskite solid solutions using the molten salt method
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Synthesis of oxide perovskite solid solutions using the molten salt method Srikanth Gopalan, Karun Mehta, and Anil V. Virkar Department of Materials Science and Engineering, 304 EMRO, University of Utah, Salt Lake City, Utah 84112 (Received 26 September 1995; accepted 2 February 1996)
The molten salt method has in the past been employed to synthesize a large number of compounds at low temperatures. In this work we report the formation of solid solutions of BaTiO3 –SrTiO3 and BaZrO3 –SrZrO3 using a molten salt eutectic of NaOH–KOH as a solvent. Alkaline earth carbonates and titanium oxide were used as precursors for the titanate system, and alkaline earth carbonates and zirconium oxide were used as precursors for the zirconate system. It was found that both systems form solid solutions throughout the composition range. The implications of these results with regards to the applicability of the molten salt method as a tool to investigate low temperature phase equilibria are discussed.
Studies of low temperature phase equilibria in the solid state are often hampered by sluggish kinetics arising from extremely low cation and/or anion diffusion coefficients. The molten salt method provides a way to circumvent this problem. Arendt1 synthesized BaFe12 O19 and SrFe12 O19 using the molten salt method. Materials such as PZT (lead zirconate titanate),2 PbNb2 O6 ,3 Pb(Fe0.5 Nb0.5 )O3 ,4,5 Ni –Zn ferrites,6 and Bi4 Ti3 O12 7 have also been synthesized using the molten salt method. In some cases the molten salt participates in the chemical reactions. It reacts with the oxide or carbonate precursor, subsequently decomposing to the product, and returns the original salt in its uncombined state. In the case of Bi4 Ti3 O12 , for instance, the component oxides react with LiCl to form an intermediate compound which then decomposes into Bi4 Ti3 O12 and LiCl.6 In other cases the molten salt acts as a mere solvent which accelerates the rate of formation of the desired compound. In addition to its utility as a synthesis tool, the molten salt method has also been used to study low temperature phase equilibria in refractory systems. Gopalan and Virkar8 showed that this method could be adapted to study low temperature phase equilibria involving decomposition reactions. They demonstrated that SrCeO3 and BaCeO3 , which have potential applications as high temperature solid electrolytes due to their high protonic conductivity, are unstable in the anticipated temperature range of application. The molten salts used in these experiments were eutectics of NaOH–KOH and KCl–LiCl. In their experiments strontium and barium cerate made by conventional high temperature calcination decomposed into the respective alkaline earth carbonates/oxides and cerium oxide in molten salt eutectics at lower temperatures. The results were independently verified by galvanic cell measurements. Recently, Yashima et al.9 studied the low temperature (1273 to 1473 K) J. Mater. Res., Vol. 11, No. 8, Aug 1996
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