Oxygen nonstoichiometry, structural phase transitions and anomalous Pr ordering for PrBa2-xLaxCu3Oz solid solution

The oxygen nonstoichiometry, structure, magnetic and calorimetric data for La doped Ba-site PrBa2-xLaxCu3Oz systems (0 less than or equal to x less th an or equal to 1 and 6.881 less than or equal to z less than or equal to 73 92) are reported. The x-dependence of the oxygen content increases monotoni cally and two structural phase transitions are observed, confirmed by powde r x-ray Rietveld analysis, from the orthorhombic 1212C-phase (space group: Pmmm) to tetragonal 1212 (P4/mmm) at x = 0.45, and then to another orthorho mbic (Cmmm) phase around x = 0.7. Magnetic susceptibility and heat capacity measurements show that the anomalous Neel temperature T-N(Pr) decreases mo notonically from 18 K for x = 0 to 2.8 K for PrBa1.3La0.7Cu3O7.236 and to b elow 2 K for x greater than or equal to 0.8 for samples prepared under the same conditions, while TN (Pr) increases with an increase of oxygen content for the same compound. The increasing Pr-O bond length observed with decre asing T-N(Pr) indicates that this unusual Pr magnetic ordering is closely r elated to the wave-function overlap between the Pr-4f orbital and eight O-2 p pi orbital in the CuO2 bi-layer. No superconductivity has been detected f or these insulating cuprates.