Oxygen nonstoichiometry in copper-oxide based superconductors and related systems: structure of nonsuperconducting Bi₂Sr_3-xY_xCu₂O_8+y (x≈0.6-1.0)

Abstract

Structural refinements of the new compounds Bi₂Sr_3-xY_xCu₂O_8+y in the range of x that gives insulating rather than superconducting behaviour (x≈0.6 and 1.0) have been obtained from single crystal X-ray and neutron powder diffraction data. Results indicate oxygen nonstoichiometry and suggest condensation into pairs or chains of the Bi---O atoms along the a axis in the Bi---O double layers. Chain-like formation can be found between the copper-oxygen sheets of all the orthorhombic superconductors including La₂CuO₄. XANES studies show that the oxidation state of copper (Cu²⁺ is not affected by yttrium substitution for strontium. The mechanism of charge compensation appears to involve the insertion of oxygen, over the ideal O₈ composition, in (rather than between) the Bi---O sheets. These materials are orthorhombic and the lattice parameters for the subcell of the x≈0.6 material are a=5.467(3), b=5.4407(6), c=30.434(5) Å, space group Amaa, Z=4. The x≈1.0 phase has a=5.467(2) b=5.426(1), and c=30.183(8) Å. The structure is essentially the same as that of superconducting Bi₂Sr_3-xCa_xCu₂O_8+y. Electron diffraction patterns for the yttrium-containing phases show an incommensurate superlattice along the a axis as seen in the calcium analog. This is due to atomic displacements that are modulated along a with amplitudes in the c direction.