Abstract
Structural, transport, magnetic, calorimetric, and neutron data were reported for the oxygen-annealed Pr <sub>1+x</sub> Ba <sub>2-x</sub> Cu <sub>3</sub> O <sub>7+y</sub> or 1212-type Cu(Ba <sub>2-x</sub> Pr <sub>x</sub> )PrCu <sub>2</sub> O <sub>7+y</sub> system (0≤x≤1, -0.11≤y≤0.31). Powder x-ray Rietveld analysis indicates that due to subtle oxygen distribution in the CuO <sub>1+y</sub> . plane for these oxygenated cuprates, two structural symmetry transitions were observed, from orthorhombic 1212C (CuO chain) O(I) phase (space group Pmmm) for 0≤x≤0.4, to tetragonal 1212 T phase (P4/mmm) for 0.4≤x≤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-Ba 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 Néel temperature T <sub>N</sub> (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 <sub>N</sub> (Pr) indicates that this unusual Pr magnetic ordering is closely correlated with the wave-function overlap between Pr-4f orbital and eight O-2p <sub>π</sub> orbital in the CuO <sub>2</sub> bilayer. ©2000 The American Physical Society.