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

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Abstract
Direct observations of CuO 2 sheet defect structures in superconducting La 2- x Sr x CuO 4 , 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 CuO 2 layers. However, unlike the open ReO 3 type-related structures, the packing density of the K 2 NiF 4 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 La 2 CuO 4 .