PR SUBSTITUTION IN Y2BA4CU6-D (N=0,1,2) - INFLUENCE ON STRUCTURE AND T-C(NO14+N)

The structural effects of the substitution of Pr for Y in Y2Ba4Cu7O15- d are studied by Rietveld analysis of neutron powder diffraction data. Comparisons are made with Pr substituted Y-123 and Y-124. A series of samples with nominal composition (Y1-xPrx)(2)Ba4Cu7O15-d, where x was varied between x = 0.0 and 1.0, was prepared by a polymerised complex method and sintered at 1 arm pressure in oxygen. The limit of solubil ity was found to be x approximate to 0.6. Susceptibility measurements show that T-c,T-onset decreases from 94.8 K to 39 K at x = 0.6. The ce ll parameters and volume increase with Pr substitution. The volume exp ansion is observed to be equal to the sum of the respective volumes ex pansions for Pr doped Y-123 and Y-124 at the same Pr content. The calc ulated bond distances show that the substitution causes an expansion o f the interplane distance around the Y(Pr) site, which is largely comp ensated by the compression of the CuO chain regions. In general, the o bserved atomic displacements in the three phases with Pr substitution, may be explained as an effect of the strain exerted by the expansion of the interplane distance. An analysis of the bond valence sums for t he metal ions was carried out to show how the internal stresses change with doping in these systems.