A/c YBa₂Cu₃O₇ boundaries: Preferential sites for the nucleation of epitaxial Y₂O₃ precipitates

Y2O3 precipitates with typical sizes between 70 and 300 nm2 have been identified by high-resolution electron microscopy and image calculations in mixed a- and c-axis oriented YBa2Cu 3O7 sputtered films. The precipitates are densely distributed (1015 cm-3), have tabular shape and grow epitaxially at boundaries between a- and c-axis oriented grains, with the (001) Y2O3 plane parallel to the a,b plane of the a-axis oriented grain and the (110) Y2O3 plane parallel to the a,b plane of the c-axis oriented grain. Their largest interfacial facet lies parallel to the a,b plane of the a-axis oriented regions. Lattice-matching arguments show that energetically this situation is the most favorable one, which explains the nucleation of precipitates at a/c boundaries.