Soluble and Volatile Precursors for the Preparation of Superconducting Films
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SOLUBLE
AND
VOLATILE PRECURSORS SUPERCONDUCTING
FOR THE FILMS
PREPARATION
OF
NANCY N. SAUER, EDUARDO GARCIA, AND ROBERT R. RYAN Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ABSTRACT Attempts to identify intermediates in an alkoxide-based solution route to films of the high temperature superconductor YBa2Cu3O7 reported by Fahrenholtz et al.[l) have led to the isolation of the first molecular barium-copper cluster, Ba 2 Cu2(OR)4(acac)4-2 HOR, R = CH2CH2OCH3. The extension of this route to the preparation of the bismuth superconductors has necessitated the investigation of bismuth alkoxide chemistry. Two separate routes to bismuth alkoxides have been examined: the metathesis of the metal halides with NaOR, (R = t-butyl, and diisopropylphenoxide) and the alcoholysis of bismuth amides, Bi(NR2)3, [R = N(SiMe3)2] The alcoholysis preparation gives high yields of several alkoxides suitable as precursors to the high Tc materials. A monomeric bismuth phenoxide, which is targeted as a volatile precursor, and a bismuth t-butoxide cluster have been isolated and stucturally characterized. Solubility and volatility studies have also been undertaken. INTRODUCTION Thin films of the new high temperature superconductors, are needed for a range of technological applications. The primary goal of this work is the identification of compounds that are suitable for solution or vapor deposition of the ceramic superconductors. To achieve this end, we have focused our research on two areas: the isolation and characterization of intermediates from proven solution routes to superconducting materials, and the design and synthesis of discrete molecular precursors to the superconductors. While a large number of reports on solution routes to superconducting films have appeared in the literature, few describe attempts to isolate and characterize intermediates. Studies have shown that critical temperature (Tc) and the critical current (Jc) in the films are highly sensitive to processing conditions. Clearly, if solution techniques are to be of value as a source of these materials, better understanding and control of the chemistry occuring in the processes will be necessary. A natural extension of this work on solution intermediates and precursors is the identification of compounds suitable for precursors for chemical vapor deposition, (CVD). RESULTS This work was begun by examining a route reported by Fahrenholtz et al. (1] which uses a dimeric copperacetylacetonate-alkoxide and barium and yttrium alkoxides as Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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soluble precursors. Attempts were made to isolate intermediates at key stages of the precursor solution preparation. One result of these studies was the isolation and structural characterization of the alkoxide cluster Ba2Cu2(acac)4(OR)40 2 HOR, R = CH2CH2OCH3, from solutions in the first stage of the thin film preparation. [2) To our knowledge this cluster represents the first example of a discrete molecular
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