STRUCTURE OF Y(PR)BA2CU4O8 - A NEUTRON-DIFFRACTION STUDY

The changes induced in the crystal structure of the high-T(c) superconductor Y1Ba2Cu4O8 by substituting Pr for Y were studied by Rietveld refinement of time-of-flight neutron powder diffraction data from a pulsed neutron source. As the Pr content increases, an overall expansion of the unit cell is observed which is attributed to the larger radius of the Pr ion. The substitution of Pr for Y also causes changes in the atomic positions, e.g. an expansion of the plane-plane distance and a contraction in the region between the planes and the double chains as well as between the double chains. Pr1Ba2Cu4O8 COUld not be synthesized, the solubility limit for Pr at the Y site in Y1Ba2Cu4O8 being approximately 80%. T(c) decreases rapidly, from 83 K at x = 0 to 8 K at x = 0.8, but is retained to a higher Pr content than in Y1Ba2Cu3O7-d.