Direct Observation and Analysis of CuO 2 Shear Defects in La 2-x Sr x CuO 4

Abstract

Direct observations of CuO₂ sheet defect structures in superconducting La_2-xSr_xCuO₄, with x in the range 0.05 ≤ x ≤ 0.15, are reported. Electron microscopy shows that extended line defects are generated in {010} planes, along crystallographic directions, by a pure shear mechanism along the edge of the octahedral copper-oxygen units. The line defects are partial screw dislocations, with characteristic displacement vectors of the type <(a/2), 0, (c/6)>, bounding the stacking faults. The existence of this type of defect demonstrates that there is an oxygen deficiency within the CuO₂ layers. However, unlike the open ReO₃ type-related structures, the packing density of the K₂NiF₄ structure necessarily requires that anion defects be accompanied by the loss of cations of the A type (lanthanum, strontium). Under identical synthesis conditions, no defects are observed in the parent compound La₂CuO₄.