In search of chemical clues: Single-crystal structural studies of PrBa 2Cu 3O 6 and PrBa 2Cu 3O 7

Abstract

Complete refinements of single-crystal X-ray diffraction intensity data for two tetragonal PrBa 2Cu 3O 6 crystals and for two detwinned orthorhombic PrBa 2Cu 3O 7 crystals, along with a more precise refinement for YBa 2Cu 3O 6, have confirmed that the praseodymium compounds are strictly isostructural with the yttrium congeners. Crystallographic data available for the superconducting orthorhombic RBa 2Cu 3O 7 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 PrBa 2Cu 3O 7 indicate no markedly unusual structural features; the crystallographic behavior of Pr corresponds to that of the trivalent ion. However, subtle structural anomalies in the PrBa 2Cu 3O x system were revealed by detailed comparisons of structural trends in RBa 2Cu 3O x systems as a function of oxygen content. The axial CuO bond distance is unusually short in both PrBa 2Cu 3O 6 and PrBa 2Cu 3O 7, 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 R 3+ suggest that in PrBa 2Cu 3O 7 the increased spatial extension of the 4f-orbitals (as compared to neighboring Nd 3+) mediates strong interactions with the CuO manifold that are capable of inhibiting superconductivity. The retention of superconductivity in Pr 1.85Ce 0.15CuO 4 may arise from significant crystal-chemical differences between the T′ — and the YBa 2Cu 3O 7-type structures: the T′ structure features (1) oxygen layers that are bonded only to Pr, not Cu, and (2) a larger R 3+ polyhedral volume that results in greater isolation of the 4f-orbitals.