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Neutron structural studies are made on (Y1-xCax)(Ba2-xLax)Cu3O7-d for compositions corresponding to x=0.0, 0.2, and 0.4 in order to evaluate any correlation between the remarkably high chemical stability of the cosubstituted samples (x=0.2 and 0.4) and the structural parameters. It is suggested that the increased chemical stability is caused by a decrease in the puckering of the Cu-O planes with increase inx, causing them to flatten, which leads to a reduction in the internal stresses existing in the perovskite lattice of this technologically important material.

The results of a systematic powder neutron study on Y1-xCaxBa2Cu3-yCoyO7±d for A (x=y=0), B (x=0;y=0.2), C (x= 0;y=0.4), D (x=y=0.2), and E (x=y=0.4) are investigated with a view to understanding the relation between the structural parameters and superconductivity. Rietveld refinements of the structures show that: (a) Co substitutes at the chain Cu(1) sites only, except for sample E, where the presence of a minor amount of Co at the planar Cu(2) site cannot be ruled out; (b) Co substitution reduced thec-parameter, which is reduced even further upon substitution of Ca at the Y-sites; (c) the occupancy factors of the chain O(1) site indicate an average coordination of 4.8 and 5.1 for Co for samples B and C, but only 4 for D and E; (d) Cu/Co(1) atoms for B and C display large thermal parameters, suggesting a displacement from their ideal centrosymmetric location; (e) the apical Cu(1)-O(4) bond lengthens upon substitution of Co in samples B and C but undergoes shortening upon substitution of Ca in the case of samples D and E. The apical Cu(2)-O(4) bond, on the other hand, shows just the opposite trend; (f) samples D and E show a reduction in the separation of CuO2 layers and their oxygen content; and (e) the bond valence of the Cu(2) ion shows the lowest value of 2.127 for the nonsuperconducting sample C.

Abstract: The effect of Ca doping in EuBaSrCu3O7−δ has been investigated by X-ray diffraction, ac susceptibility, and electrical resistivity measurements. X-Ray diffraction analyses of the samples showed that up to 30 at.% Ca can be substituted for Sr in EuBaSrCu3O7−δ, which has orthorhombic symmetry. The orthorhombicity decreases with increase inx, in EuBaSr1−xCax Cu3O7−δ, the material becoming tetragonal atx=0.2. Ac susceptibility and dc electrical resistivity measurements as function of temperature from 10 to 300 K showed that the superconducting transition temperatureT c decreases monotonically from 84 to 75K as the Ca concentrationx increases from 0.0 to 0.3 The decrease inT c can be attributed to the structural change taking place in EuBaSrCu3O7 as a consequence of replacement of Sr by Ca.