Ordered-defect structure in epitaxialYBa2Cu3O7−xthin films

An ordered defect structure in superconducting Y-Ba-Cu-O thin films has been characterized by both x-ray diffraction and transmission electron microscopy. The defect structure, which is observed growing epitaxially within the grains of normal $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ structure, has the diffraction characteristics of a distinct phase whose volume fraction can be correlated with changes in film composition. The diffraction characteristics are consistent with an orthorhombic unit cell ($a\ensuremath{\cong}b=3.86$ \AA{}, $c=27.19$ \AA{}) with space group Ammm. These are the characteristics to be expected from a structure in which extra copper-oxygen layers create fault planes, which are inserted at every unit cell in the parent $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ structure. The composition expected for the pure phase is ${\mathrm{Y}}_{2}{\mathrm{Ba}}_{4}{\mathrm{Cu}}_{8}{\mathrm{O}}_{20\ensuremath{-}x}$. Preliminary transport measurements on films containing this extra phase are characterized by lower normal-state resistances and a lower Hall constant.