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 YBa₂Cu₃O₇ 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.