PROCESSING AND CHARACTERIZATION OF THIN-FILMS OF THE 2-LAYER SUPERCONDUCTING PHASE IN THE BI-SR-CA-CU-O SYSTEM - EVIDENCE FOR SOLID-SOLUTION

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 T(c) 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-degrees-C to an upper, solidus (or peritectic) temperature (780-degrees to 840-degrees-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.