Effect of phase separation in metal carboxylate gels on perovskite lead magnesium niobate crystallization
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Effect of phase separation in metal carboxylate gels on perovskite lead magnesium niobate crystallization Yeshwanth Narendar and Gary L. Messing Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802-4801 (Received 28 October 1997; accepted 30 June 1999)
Phase separation during the synthesis and decomposition of lead magnesium niobate (PMN)–ethylenediaminetetraacetic acid (EDTA) and PMN–citrate gels strongly affects perovskite Pb(Mg1/3Nb2/3)O3 phase formation. The PMN–EDTA gel was synthesized from a solution containing Pb–EDTA, Mg–EDTA, and peroxo–citrato–niobium complexes at pH 8. Pb–EDTA precipitation was avoided by using Pb:EDTA in the mole ratio >1:2.5 and flash pyrolyzing the PMN–EDTA solution at 225 °C. Consequently, the PMN yield increased from 80 to 98 wt%. The sequential decomposition of Pb–EDTA, Mg–EDTA, and peroxo–citrato–Nb in the PMN–EDTA precursor in 1 vol% O2 leads to phase separation of Pb and PbO and thus lowers the PMN yield to 92 wt%. At PO2 > 2.5 vol% the Pb, Mg, and Nb complexes cothermolyze to form 艌97 % perovskite PMN. The presence of a heterometallic citrato–Pb–Mg–Nb complex in PMN–citrate leads to oxygen partial pressure independent codecomposition of the Pb, Mg, and Nb complexes. Accordingly, PMN yields of 艌96 wt% were obtained from the PMN–citrate precursor for oxygen partial pressures between 1 and 5 vol%.
I. INTRODUCTION
The formation temperature, chemical homogeneity, and phase purity of multicomponent oxides depend on the cation homogeneity in the oxide precursor. Much research has been directed at the synthesis of precursors with molecular level mixing of the constituent metal cations. One of the simplest strategies is to fix the scale of metal ion homogeneity in solution by evaporating the solvent and forming an amorphous glassy solid.1,2 However, it is difficult to obtain a glassy precursor from a solution containing simple inorganic salts such as acetates or nitrates because of limitations on the needed pH range and metal ion concentrations.3,4 In contrast to inorganic salts, homogeneous molecular scale metal oxide precursors can be obtained by complexing metal ions with organic chelating agents.1,5 Typically chelating agents, with high metal chelate formation constants, such as citric acid and ethylenediaminetetraacetic acid (EDTA), are used to form metal carboxylate gels. Chelating agents expand the range of possible pH values and metal ion concentrations for synthesizing homogeneous glassy precursors and thus are very versatile precursors.4,6,7 To completely avoid metal ion precipitation in metal carboxylate gels, it is important to complex all metal ions with the chelating agent prior to gel formation. Using the equilibrium constants for metal chelate J. Mater. Res., Vol. 14, No. 10, Oct 1999
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complexes, the chelating agent to metal ratio, the pH range, and the temperature for forming stable stoichiometric meta
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