TEM OBSERVATION OF INTERGROWTH STRUCTURES IN THE HGBA2CA2CU3OY CERAMIC SUPERCONDUCTOR

Intergrowth structures, which occur in the HgBa2Ca2Cu3Oy ceramic super conductor, 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 in tergrowth structures are respectively: [1 1 0](Hg-1212) parallel to[1 1 0](Hg-1223), {0 0 1}(Hg-1212) parallel to{0 0 1}(Hg-1223); and [1 1 0](Hg-1223) parallel to[1 1 0](Hg-1234), {0 0 1}(Hg-1223) parallel to{ 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 s hear 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 t he numbers of matching slabs, i.e. f(i) = N-i/N, where N-i is the numb er 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) have minima at f(i) = 1.2498 and f(i) = 0.8272 for inter growth structures of Hg-1212/Hg-1223 and Hg-1234/Hg-1223, respectively . As was observed from the TEM micrograph, two immobile dislocations c an 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 t hese immobile dislocations is presented.