Electron microscopy of superconducting Pb₂Sr₂Y_1-xCa_xCu₃O₈

Electron microscope results are presented on Pb₂Sr₂Y_1-xCa_xCu₃O_8+∂ (for x=0 and x=0.5, ∂≈O) with a unit cell of a=5.40, b=5.43 and c=15.75 Å. Very weak but well defined h, k, reflections for h+k odd are always present and the crystals are twinned on (110) planes. This twinning plane is at 45° to that found in YBa₂Cu₃O₇. These observations indicate that the orthorhombic unit cell {check mark}2a_p × {check mark}2a_p × c (where a_p is the perovskite unit cell) induced by a small displacement of the oxygen atoms in the PbO layers is well ordered and not C centered. No additional long range superstructure was found in the samples studied. A low density of (001) twist boundaries and [110] tilt boundaries are present. No intergrowths or stacking faults in the basic cation lattice were ever observed. However low magnification images show that a few isolated layers have a slightly higher scattering density than their neighbours, while retaining the same c spacing. This can be interpreted as the presence of additional oxygen in the PbO-Cu-PbO layers concerned.