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 (YBa2Cu3O7−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 crystals2–3 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 diffraction4,5 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 Tc 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.