X-ray structural studies of the superconducting La3.5-x-yNdyCa2xBa3.5-xCu7Oz system

We have investigated the influence of the incorporation of Ca and Nd ions o n the structural and superconducting properties of the La3-5-xd-yNdyCa2xBa3 .5-xCu7Oz (0 less than or equal to x = y less than or equal to 0.60) system by Rietveld refinement of x-ray diffraction patterns of seven samples, i.e . x = y = 0.00, 0.10, 0.20, 0.30, 0.40, 0.50 and 0.60 together with a.c. su sceptibility and oxygen content measurements. X-ray data confirm the occurr ence of a single-phase tetragonal structure with a space group of P4/mmm fo r all the seven samples and all Nd ions exclusively occupy La sites. Ca sub stitutes onto the La sires with a concomitant displacement of La onto the P a sites for x = y less than or equal to 0.2 and for x = y greater than or e qual to 0.3; most of the Ca, greater than or equal to 80%, enters La sites with some fraction occupying Ba sites. Samples with 0.20 less than or equal to x = y less than or equal to 0.60 are superconducting and T-c is between 42 and 80 K. With increasing x = y, both the hole concentration in the Cu- 2-O layers and T-c increase to a maximum value of 0.309 and 80 K respective ly at x = y = 0.6. An approximate correlation between T-c, and the hole con centration is suggested.