Structural properties of the superconductor LaBa2Cu3−yO7−z in the solid solution system La1+xBa2−xCu3−yO7−z

Abstract

We present time-of-flight (TOF) neutron-powder-diffraction results of four samples in the solid-solution orthorhombic La1+xBa2−xCu3−yO7−z system. The data for all of the samples were taken at room temperature. The average crystal structures of LaBa2Cu2.90O6.82 (Tc=45 K), La1.56Ba1.44Cu2.86BO6.98 (nonsuperconductor), and the impurity-free LaBa2Cu3O6.85 (Tc=93 K) were refined by the Rietveld analysis which includes the refinement of the occupancies of the La and/or Ba atoms. The remaining sample was imhomogeneous LaBa2Cu3−yO7−z (Tc=85 K). The obtained structures are essentially isomorphous with the oxygen-deficient triperovskite structure (space group Pmmm). The refined results show that the subunit around the La at the 1h site is well ordered as is shown in the 90-K superconductor YBa2Cu3O7. The valence-sum calculation indicates that Cu(1) and Cu(2) atoms are dominantly divalent in both the 45-K and 93-K superconductors, LaBa2Cu2.90O6.82 and LaBa2Cu3O6.85, while there exists trivalent copper at the Cu(1) site in nonsuperconducting La1.56Ba1.44Cu2.86BO6.98. The difference and resemblance between the crystal structures of 45-K and 93-K superconducting samples were also clarified based on the obtained microscopic crystal parameters with respect to the structure of the two-dimensional CuO2 network, where the parameter of the degree of coplanarity was introduced. In the structure of the solid solution, it is also remarked that the excess La ion occupies the site of a Ba ion and most of the La ions have a larger ionic radius with twelve coordination numbers to oxygen atoms in contrast to eight coordination numbers of the La ion at the 1h site. It is shown to be quite significant to analyze the TOF powder-diffraction data for both material characterization and the study of the physical properties of superconducting oxides in these solid-solution systems.

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