IN SEARCH OF CHEMICAL CLUES - SINGLE-CRYSTAL STRUCTURAL STUDIES OF PRBA2CU3O6 AND PRBA2CU3O7

Complete refinements of single-crystal X-ray diffraction intensity data for two tetragonal PrBa2Cu3O6 crystals and for two detwinned orthorhombic PrBa2Cu3O7 crystals, along with a more precise refinement for YBa2Cu3O6, have confirmed that the praseodymium compounds are strictly isostructural with the yttrium congeners. Crystallographic data available for the superconducting orthorhombic RBa2Cu3O7 analogs (R = Y, rare earth) were used to establish overall structural trends as a function of lanthanide size. In comparison with these trends, the present results obtained for orthorhombic PrBa2Cu3O7 indicate no markedly unusual structural features; the crystallographic behavior of Pr corresponds to that of the trivalent ion. However, subtle structural anomalies in the PrBa2Cu3Ox system were revealed by detailed comparisons of structural trends in RBa2Cu3Ox systems as a function of oxygen content. The axial Cu-O bond distance is unusually short in both PrBa2Cu3O6 and PrBa2Cu3O7, indicating stronger chemical linkage of the chains and planes as compared to the superconducting systems. Trends in the bond-valence sums and polyhedral volumes about R3+ suggest that in PrBa2Cu3O7 the increased spatial extension of the 4f-orbitals (as compared to neighboring Nd3+) mediates strong interactions with the Cu-O manifold that are capable of inhibiting superconductivity. The retention of superconductivity in Pr1.85Ce0.15CuO4 may arise from significant crystal-chemical differences between the T'- and the YBa2Cu3O7-type structures: the T' structure features (1) oxygen layers that are bonded only to Pr, not Cu, and (2) a larger R3+ polyhedral volume that results in greater isolation of the 4f-orbitals.