Phase competition between Y2BaCuO5 and Y2O3 precipitates in Y-rich YBCO thin films

Segregation and competition of Y-based impurities in Y-rich biepitaxial YBCO samples is investigated. The analysis of distribution and microstructure of the precipitates is performed by high resolution electron microscopy (HREM). Y2O3 precipitates are found in (103) YBCO films grown on (110) SrTiO3 (STO) substrates. On the same biepitaxial samples, Y2BaCuO5, (Y211) precipitates are found in (001) YBCO films deposited on an intermediate (110) MgO seed layer. Agglomeration of Y211 precipitates is observed near the edge in the MgO seed layer. The experimental data are discussed in the context of phase competition, nucleation, epitaxy and surface migration in the Y-Ba-Cu-O system. It is deduced that the change of supersaturation and surface energy, also governed by the favorable epitaxy of the secondary phase with the substrate or the YBCO matrix, plays a major role in determining the segregation of either Y211 or Y2O3 precipitates. (C) 1999 Elsevier Science B.V. All rights reserved.