Ordered Structures in YBa sub 2 Cu sub 3 O/sub 7-Y/, La/sub 2-X/Sr/sub X/CuO/sub 4-Par. Delta/ and Related Perovskites

摘要

Electron microscopy and electron diffraction studies have been performed on the superconducting oxides YBa sub 2 Cu sub 3 O/sub 7-y/, GdBa sub 2 Cu sub 3 O/sub 7-y/ and La/sub 2-x/Sr/sub x/CuO/sub 4-par. delta/ as well as the related perovskites La sub 2 CuO sub 4 , Eu sub 2 CuO sub 4 and Gd sub 2 CuO sub 4 . Extra reflections are commonly observed in all the cases. For example, in the 123 compounds, in situ heating leads to transformations from orthorhombic to tetragonal with a loss of twin structure; on cooling the oxygen vacancies re-order in the basal plane to give 1/3(100)* or 1/4(110)* diffraction spots. In tetragonal Eu sub 2 CuO sub 4 and Gd sub 2 CuO sub 4 , the (001) diffraction patterns often have extra spots at 1/2(110)* or at 1/4(110)* positions. Extra spots are also observed in La sub 2 CuO sub 4 (orthorhombic) but this is probably due to the space group being primitive rather than centered. These observations are discussed in terms of the ordering of oxygen vacancies in the Cu-O planes of the various perovskite structures. 6 figs. (ERA citation 13:024679)