Fast Solution Precursor Synthesis of the 2223 Phase: The Role of Lead in the Reaction Pathway
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ABSTRACT Valuable mechanistic information, about the reaction pathway in the formation of the 2223 phase, has been obtained by the use of a solution synthesis route. The presence of lead in the starting mixture has remarkable effects on the nature and stability of the different intermediates: i) the grain size and morphology of Bi 2CuO 4 is severely changed, ii) calcium carbonate is partially decomposed to yield the calcium plumbate, iii) the 2201 phase results stabilized against the formation of the collapsedphase Bi 17 Sr16Cu 70 49 _8,iv) the melting point of the intermediate 2212 is lowered by about 10 'C. All these effects, due to lead substitution, contribute to promote the formation of the 2223 phase, controlling also the morphology and properties of the final product.
INTRODUCTION It is well established that the thermodynamic stability regions of the entropy stabilized bismuth cuprate superconductors (Bi2Sr2Ca-lCUnO 4 2zn) become narrower as n increases. 1 In fact, whereas the n=2 member can be easily obtained as single phase, the isolation of pure and stoichiometric n=3 (2223) phase only can be achieved under extreme conditions. Although many compositional modifications have been proposed, only a partial substitution of lead in the bismuth sites seems to improve critically the formation of the 2223 phase.2 Indeed, very few successful syntheses have been reported without addition of a certain amount of lead and, in these cases, extremely long thermal treatments (30 days) and overstoichiometric quantities of copper are required. 3 The structures of these compounds can be viewed as pairs of BiO1 +8 layers, which form a rock-salt like sheet, and a perovskite slab sandwiched between each two of these layers. The perovskite layer determines, in essence, the a- and b- axes, this resulting in an inconsistency between the Bi-O distances and that what would be expected from the ionic radii. 4 The structural role of the Pb substitution, resulting in a stabilization of the BiO layers, has been studied in detail by means of HREM and ED. 5,6 . Goodman and coworkers propose a Pb 2 +/Pb4 + coupled substitution in different bismuth sites labeled as Bi(4) and Bi(l), respectively. 7 This substitution modifies significantly the modulation period and geometrical features of the modulation wave, which can contribute to minimize the misfit between the rock-salt and perovskite layers. We have described recently very efficient synthetic procedures for obtaining the 2223 phase. 8 Besides the concomitant improvement of the synthesis, these methods allow identification of all the intermediate compounds, which improves the understanding of the role of lead in the successive reaction steps. In this work we describe the different steps in the formation of the 2223 phase, paying special attention to the identification of the different phases that appear during the whole process. The morphology and magnetic properties of the final products are also discussed.
377 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
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