Synthesis and characterization of barium bis(citrato) oxozirconate(IV) tetrahydrate: A new molecular precursor for fine

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Barium metazirconate (BaZrO3) fine powder has been produced by thermally decomposing a molecular precursor, barium bis(citrato)oxozirconate(iv) tetrahydrate at about 700 °C. The precursor, Ba[ZrO(C 6 H 6 O 7 ) 2 ] • 4H 2 O (BZO) has been synthesized and characterized by employing a combination of spectroscopic and thermoanalytical techniques. The precursor undergoes thermal decomposition in three major stages: (i) dehydration to give an anhydrous barium zirconyl citrate, (ii) decomposition of the anhydrous citrate in a multistep process to form an ionic oxycarbonate intermediate, Ba 2 Zr 2 05C0 3 , and (iii) decomposition of the oxycarbonate to produce BaZrO 3 fine powder. The particle size of the resultant BaZrO 3 is about 0.2 ^ m , and the surface area is found to be 4.0 m2 g" 1 .

I. INTRODUCTION Barium metazirconate forms a complete range of solid solution with alkaline-earth and lead titanates of perovskite family to give a wide variety and large number of oxide perovskites. The resultant compounds exhibit a wide spectrum of physical properties and find immense use as electroceramic materials on appropriate processing.1"4 Because of the high melting point and chemical stability, BaZrO 3 has been considered to be a promising candidate for special refractory applications.5 Of late, BaZrO 3 has been found to be a good substrate for depositing superconducting oxide films due to its structural and chemical stability at elevated temperatures.6 A recent high temperature x-ray diffraction study has shown that BaZrO 3 remains cubic till 1375 K7 and the absence of polymorphic transformation below this temperature signifies the suitability of this material as a substrate. Thus, there is a growing interest in this material to exploit its potential for a variety of practical applications. The first step in such an attempt is to synthesize a strictly stoichiometric, fine particle BaZrO 3 . It is well known that the performance of ceramics depends to a large extent on the powder characteristics and the processing conditions. Therefore, it is desirable to employ chemical methods for synthesizing ceramic powders to eventually achieve optimum performance characteristics on processing. A great degree of reproducibility in the

"^Author to whom correspondence should be addressed. Presently at Materials Science Research Centre, Indian Institute of Technology, Madras 600 036, India. J. Mater. Res., Vol. 9, No. 9, Sep 1994

physical properties of the compacts processed through chemical routes has been well established for electroceramic compounds.8 Various chemical routes are available to synthesize mixed metal oxides.8"19 Among these, molecular precursor techniques are efficient to synthesize mixed metal oxides at relatively low temperatures.8'10"12'17"20 This method enables an intimate molecular level mixing of the metal ions of desired stoichiometry in the precursor. The precursor on thermal decomposition produces the corresponding mixed metal oxide at remarkably low temperatures. Sheinkman et al.20 have employed barium zirconyl