X-Ray Absorption Spectroscopy and Local Structure Studies of Some High Critical Temperature Superconductors.

This thesis is divided into two parts. The first half is focused on the XAFS (x-ray-absorption fine-structure) technique. A technical problem, called an "XAFS glitch", is investigated; excellent agreement between the simulation and the experimental pinhole glitch is achieved. The study shows that the XAFS glitch is induced by the coupling of sample non-uniformity with the vertical movement of the crystal glitch in space as the energy is changed. Multielectron excitations involving deep and shallow core levels have been identified for the first time in the XAFS energy range for crystalline samples, RbBr and beta-PbO_2. We have proposed an iterative background extraction and removal procedure. The positions of steps in the extracted background are in excellent agreement with the Z + 1 approximation. Similar features are also found in Pt, Au, Tl_2O _3 and other materials. We have compared about 30 isolated experimental pair standards with those generated by the FEFF5 code. In general, good agreement is achieved, with typical errors of 0.005 A for nearest neighbor distances. The effective amplitude reduction factor, S_sp{o }{2}_{eff} and the E_ o shift for the calculated XAFS standards are also given; the former can vary by +/-20%. The second half of the thesis is the local structure studies of some high T_ c superconductors using the XAFS technique. We have investigated the local structure about Cu and Co atoms in YBa_2(Cu_{1-x}Co_ x)_3O _{7-delta} (x=0.05~ 0.30), processed with different thermal treatments. We find no evidence for the proposed movement of Co to the Cu(2) site during the thermal treatment. The analysis indicates that Co occupies a strongly distorted Cu(1) site in which CoO_ z clusters are displaced relative to the rest of the lattice. The local structure about the Cu, Ba, and Tl atoms in Tl_2Ba_2CuO _ y has been studied. The results clearly show that the Tl and O(3) atoms are more disordered than suggested by the average crystal structure. The detailed data analyses indicate that the Tl atom is displaced along the <110> direction from its ideal 4e site by about 0.11 A and the O(3) atom is shifted from the 4e site by about 0.53 A roughly along the <100 > direction. These displacements are correlated.