TEM observation of intergrowth structures in the HgBa2Ca2Cu3Oy ceramic superconductor

Abstract

Intergrowth structures, which occur in the HgBa2Ca2Cu3Oy ceramic superconductor, were observed by means of transmission electron microscopy (TEM). There are at least two types of intergrowth structure: Hg-1212/Hg-1223 and Hg-1223/Hg-1234. The congruence relationships for these intergrowth structures are respectively: 〈1 1 0〉Hg-1212|〈1 1 0〉Hg-1223, {0 0 1}Hg-1212|{0 0 1}Hg-1223; and 〈1 1 0〉Hg-1223|〈1 1 0〉Hg-1234, {0 0 1}Hg-1223|{0 0 1}Hg-1234. For a stable intergrowth structure, the total strain energy density should have a minimum value. The strain energy density consists of two parts: the tensile strain energy density Ut and the shear strain energy density Us. The tensile strain energy density Ut stored in the crystal can be expressed in terms of the ratio between the numbers of matching slabs, i.e. fi = Ni/N, where Ni is the number of slabs corresponding to Hg-12(ni−1)ni, and N is the number of slabs corresponding to the matrix, i.e. Hg-12(n−1)n. The plots of Ut vs. fi = 1.2498 and fi = 0.8272 for intergrowth structures of Hg-1212/Hg-1223 and Hg-1234/Hg-1223, respectively. As was observed from the TEM micrograph, two immobile dislocations can be created by an appropriate combination of intergrowth structures of Hg-1212/Hg-1223 and Hg-1223/Hg-1234. A model for the formation of these immobile dislocations is presented. Copyright © 1995 Published by Elsevier B.V.