Heavy atom disorder in the high T_c superconductor Ba₂YCu₃O_7-δ studied by means of electron microscopy and electron diffraction

It is shown that on rapid cooling of the compound Ba₂YCu₃O_7-δ one obtains a highly disordered material in which the perovskite framework is conserved but in which barium and yttrium atoms exhibit a high degree of disorder. This leads to lattice deformations, which can be visualized in the electron microscope. The electron diffraction patterns also reveal the disorder. As-quenched specimens are no longer high T_c superconductors, but the superconducting properties can be recovered by an adequate heat treatment whereby the vacancies become ordered.