Structural studies of the superconducting La 3.5− x− yY yCa 2 xBa 3.5− xCu 7O z system

Abstract

We have investigated the influence of incorporation of Ca and Y ions on the structural and superconducting properties of La 3.5− x− y Y y Ca 2 x Ba 3.5− x Cu 7O z (0.0≤ x= y≤0.7) system by Rietveld refinement using the neutron diffraction data on five samples viz. 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 measurements. Samples with 0.20≤ x= y≤0.70 are superconducting and T c R=0 is between 56 and 79 K. Neutron data confirms (i) the occurrence of a single phase tetragonal structure with space group P4/ mmm for all the five samples, (ii) Ca and Y ions substitute onto the La sites, with concomitant displacement of La on to Ba sites, and (iii) increasing x= y from 0.0 to 0.50 decreases La–O(3) bond length from 2.516 to 2.446 Å and increases Cu(2)–O(2) bond length from 2.200 to 2.292 Å with corresponding increase in T c from 0 to 79 K due to increasing occupancy of Y/Ca at La site. A correlation exists between the Cu(2)–O(2) and La–O(3) bond lengths and the measured 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 Ba sites. Such cation disordering between Ca and La,Ba may give different oxygen contents obtained from neutron data and iodometry.