Structural studies of the superconducting La3.5-x-yYyCa2xBa3.5-xCu7Oz system

We have investigated the influence of incorporation of Ca and Y ions on the structural and superconducting properties of La3.5-x-yYyCa2xBa3.5-xCu7Oz ( 0.0 less than or equal to x = y less than or equal to 0.7) system by Rietve ld refinement using the neutron diffraction data on five samples vit. x = y = 0.0 (#1), 0.20 (#2), 0.40 (#3), 0.50 (#4) and 0.70 (#5) along with X-ray diffraction (XRD), resistivity, AC susceptibility and oxygen-content measu rements. Samples with 0.20 I x = y I 0.70 are superconducting and T-c(R=0) is between 56 and 79 K. Neutron data confirms (i) the occurrence of a singl e phase tetragonal structure with space group Pd/mmm for all the five sampl es, (ii) Ca and Y ions substitute onto the La sites, with concomitant displ acement of La on to Pa sites, and (iii) increasing x = y from 0.0 to 0.50 d ecreases La-O(3) bond length from 2.516 to 2.446 Angstrom and increases Cu( 2)-O(2) bond length from 2.200 to 2.292 Angstrom with corresponding increas e in T-c from 0 to 79 K due to increasing occupancy of Y/Ca at La site. A c orrelation exists between the Cu(2)-O(2) and La-O(3) bond lengths and the m easured T-c values of five samples. Rietveld analysis of XRD data for x = y = 0.50 (#4) sample clearly shows the occupancy of Ca ions in the La sites and to small extent 0.03 in Pa sites. Such cation disordering between Ca an d La,Ba may give different oxygen contents obtained from neutron data and i odometry. (C) 1999 Published by Elsevier Science B.V. All rights reserved.