Orientational dependent x-ray absorption fine structure (XAFS) measurements on the local structure of the La2CuO4 based high Tc superconductors La2-xSrxCuO4, La2-xBaxCuO4, La1.6-xSrxNd0.4CuO4, La1.85Sr0.15Cu1-yNiyO4, find both displacement and orientational disorder induced by doping (Sr,Ba) and alloying (Nd,Ni) atoms. The displacement disorder is in the first few neighboring atoms to the perturbing atom while the orientational disorder is of
nanometer-sized regions which have disordering of the tilt angles of their CuO6 octahedra with respect to neighboring regions. This disorder affects both the normal and superconducting properties of the materials. It is found that disorder induced in the Cu-O2 planes has the greatest detrimental effect on Tc and conductivity.
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