TEM observation of intergrowth structures in the HgBa 2Ca 2Cu 3O y ceramic superconductor

Abstract

Intergrowth structures, which occur in the HgBa 2Ca 2Cu 3O y 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 U t and the shear strain energy density U s. The tensile strain energy density U t stored in the crystal can be expressed in terms of the ratio between the numbers of matching slabs, i.e. f i = N i/ N, where N i is the number of slabs corresponding to Hg-12( n i−1) n i, and N is the number of slabs corresponding to the matrix, i.e. Hg-12( n−1) n. The plots of U t vs. f i = 1.2498 and f i = 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.