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 $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ 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 ($a\cong b=3.86\mathrm{}$ Å, $c=27.19\mathrm{}$ Å) 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 $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ structure. The composition expected for the pure phase is ${\mathrm{Y}}_2{\mathrm{Ba}}_4{\mathrm{Cu}}_8{\mathrm{O}}_{20-x}$. Preliminary transport measurements on films containing this extra phase are characterized by lower normal-state resistances and a lower Hall constant.