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A TEM investigation of high T(c) superconductors and related perovskites

by Roy, Tapan

Abstract (Summary)
The structure and microstructure of the new perovskite superconductors, MBa2Cu3O7 (where M = Y, Eu, Gd; known as 123) and La 2-xSr xCuO4 (known as 214), and the related perovskites, Eu2CuO4, Gd2CuO4 and La2CuO4 have been investigated using transmission electron microscopy. Various experiments were performed in situ in the microscope on bulk ion-milled specimens using heating and cooling holders. Twins are the most prominent microstructural features in both the 123 and 214 type compounds. These twins are formed to accommodate the strains arising from the difference in the a and b parameters below the tetragonal to orthorhombic transformation temperature. The b/a ratio of YCa2Cu3O7 and La2CuO4 has been determined over a range of temperatures. The b/a ratio in 123 materials reaches a peak at ?150K below the transformation temperature and remains fairly constant below that temperature but in 214 compounds, the b/a ratio increases monotonically. In 123 materials, the orthorhombic distortion arises from the ordering of the O atoms to form Cu-O chains along the b axis whereas in the 214 materials the orthorhombic distortion occurs by the tilting of the CuO6 octahedra that make up the structure. The electro n beam when condensed is intense enough to displace oxygen atoms in 123 materials leaving behind vacancies. These oxygen vacancies order themselves and this is observed in the diffraction pattern as superlattice reflections. Two basic types of ordering were observed – the first type with extra reflections at the 1/3(100) and 2/3(100)* type positions and the second with extra reflections at the 1/4(110)*, 1/2(110)* and 3/4(110)* type positions. The two types of ordering that were observed were modeled in terms of superlattices with lattice parameters of 3a T x a T x c T and surd2a T x 2surd2a T x c T respectively. Sometimes the 123 specimen can be melted incongruently with an intense electron beam and the products of melting are found to be BaCuO2, Y2BaCuO5 and Cu2O. Using energy considerations, the twin boundary spacing (?) is related to the colony size(L) by the expression ? = varepsilon-1 (2.?.L/?) 0.5. This expression has been verified from measurements of ? and L in bright field micrographs. From the slope of a ? vs L1/2 plot, ? can be calculated from the expression: ? = 0.5(slope)2.varepsilon2*? and the values obtained are 2.2mJ/m^2 for 123 materials and 4.8 mJ/m^2 for La2CuO4
Bibliographical Information:

Advisor:

School:Case Western Reserve University

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:tem superconductors perovskites

ISBN:

Date of Publication:01/01/1990

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