Variation of anomalous Pr ordering and crystal symmetry for the oxygenated Pr1+xBa2-xCu3O7+y system

Structural, transport, magnetic, calorimetric, and neutron data were reported for the oxygen-annealed Pr1+xBa2-xCu3O7+y or 1212-type Cu(Ba2-xPrx)PrCu2O7+y system (0 less than or equal to x less than or equal to 1, -0.11 less than or equal to y less than or equal to 0.31). Powder x-ray Rietveld analysis indicates that due to subtle oxygen distribution in the CuO1+y plane for these oxygenated cuprates, two structural symmetry transitions were observed, from orthorhombic 1212C (CuO chain) O(I) phase (space group Pmmm) for 0 less than or equal to x less than or equal to 0.4, to tetragonal 1212 T phase (P4/mmm) for 0.4 less than or equal to x less than or equal to 0.65, and then to a different type of orthorhombic 1212 O(II) phase (Cmmm) for 0.65<x<1. Resistivity data indicate that Pr-in-Pa is not favorable for metallic state and no superconductivity can be detected for these insulating cuprates. Magnetic-susceptibility, heat-capacity, and neutron-diffraction data show that, regardless of the structural transitions, the anomalous Neel temperature T-N(Pr) decreases monotonically and smoothly from 18 K for x=0 to 2.5 K for x=0.8, with the same c-axis antiferromagnetic Pr spin alignment. The increasing Pr-O bond length observed with decreasing T-N(Pr) indicates that this unusual Pr magnetic ordering is closely correlated with the wave-function overlap between Pr-4f orbital and eight 0-2p(pi) orbital in the CuO, bilayer.