Crystal structure of the YBa₂Cu₃O₇ superconductor by single-crystal X-ray diffraction

Abstract

Just after Wu et al.¹ reported superconductivity above 90 K in barium, yttrium and copper ternary oxides, the formula (YBa₂Cu₃O_7−x) and the structural arrangement (oxygen-deficient tripled-perovskite-type structure) of the superconducting phase was proposed by different groups. As theoretical proposals depend crucially on the details of the atomic arrangement reducing the dimensionality of the system, a number of structural determinations were carried out by X-ray, neutron and electron diffraction. Although these studies established the structure of the metal framework (the ordering of Ba and Y in the sequence Y–Ba–Ba being responsible for tripling the crystallographic c axis), discrepancies persist regarding the position of the oxygen vacancies. X-ray experiments on single crystals^2–3 have reported that the oxygen vacancies are distributed in an ordered way in the plane containing Y atoms (z = 1/2) and statistically in the plane at z = 0. High-resolution neutron-powder diffraction^4,5 showed, on the other hand, a reduction in the structure dimensionality. The oxygen vacancies in the plane at z = 0 are ordered along the a-axis, so that one-dimensional chains of square-coordinated copper ions parallel to the b–c plane are formed. These results, confirmed by electron diffraction⁶, justify the reduction of crystal symmetry from tetragonal to orthorhombic observed in several studies. Structural differences between X-ray and neutron diffraction studies were attributed⁴ to the fact that single crystals used in X-ray measurements were probably highly twinned. But neutron experiments need large amounts of material, increasing the possibility of the samples being inhomogeneous. Here we report X-ray structure determination on a YBa₂Cu₃O₇ single crystal showing a T_c above 90 K. The results, in agreement with neutron-powder diffraction, confirm the complete ordering of the oxygen vacancies leading to the one-dimensional chains.