Twin formation and structural modulations in orthorhombic and tetragonal YBa₂(Cu_1-xCo_x)₃O_7-δ

Abstract

Abstract The microstructure of YBa2(Cu1−xCox)3O7−δ, prepared by the standard ceramic method, shows lamellar twin structures with decreasing spacings between twin walls with increasing Co content for x≲0·02, developing into {110}-type cross-hatched ‘tweed’ modulation for x≳0·02. Several wall junctions are found for x=0·02. The structural phase transition between macroscopically orthorhombic and tetragonal material occurs at x≈0·025; structural modulations (λ≈20Å) persist in the samples with high Co content (x>0·25). The modulations lead to a considerable broadening of the X-ray lines affected by orthorhombic splitting, and show maximum amplitude at the critical composition x≈0·025.