Processing and Characterization of Thin Films of the Two‐Layer Superconducting Phase in the Bi─Sr─Ca─Cu─O System: Evidence for Solid Solution

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Abstract
Thin films in the Bi─Sr─Ca─Cu─O system have been synthesized from liquid ethylhexanoate precursors by spin pyrolysis. An extensive solid‐solution range was found for the two‐Cu‐layer phase through the study of c‐axis‐oriented, single‐phase thin films fabricated on single‐crystal, (100)‐oriented, MgO substrates. All two‐layer compositions were excess in Bi and deficient in Sr + Ca relative to the ideal 2212 composition and showed an overall cation deficiency. The solidus temperature and c lattice parameter were found to vary systematically with composition. Sharp superconductive transitions were obtained in the case of a number of different compositions with Tc varying between 72 and 84 K. Evidence for significant compositional heterogeneities within single‐phase two‐layer thin films was found and the implications for superconductivity are discussed. Compositions within the solid‐solution range gave single‐phase, c‐axis‐oriented films over a wide temperature range extending from 730°C to an upper, solidus (or peritectic) temperature (780° to 840°C) which is dependent on the initial starting composition. A model has been developed that describes the formation of the two‐layer phase from a fugitive liquid.