Structure of the 100 K Superconductor Ba₂YCu₃O₇ between (5 ÷ 300) K by Neutron Powder Diffraction

Recently Siegrist et al. proposed a structure for a high-T_c superconductor Ba₂YCu₃O_(9-∂) based on an orthorhombic (a = a_p, b = a_p, c = 3a_p) perovskitelike model containing Ba and Y cations ordered over the A-sites of the ABO₃ structure. This ordering is responsible for the tripling of the c-axis. Half of the oxygen vacancies (at z = 1/2) are ordered, while the other half (at z = 0) are disordered over two sites. Using profile refinement of high-resolution neutron powder data at six different temperatures between 5 K and 300 K, we have refined the structure of a pure and well-characterised powder sample with onset of superconductivity at 100 K. At all temperatures we confirmed the previous model except that in our structure, all oxygen vacancies are ordered. Two-thirds of the copper cations have a pyramidal coordination and one-third has a square coordination. In our structure all squares are parallel to the (b, c)-plane, while in the one reported by Siegrist et al. the squares are disorderedly parallel to either the (a, c)- or (b, c)-plane. The difference between the two models is probably due to the fact that the single crystal used in the earlier work was highly twinned. Empirical calculations of the copper valences show that the Cu⁺⁺⁺ cations are almost equally distributed over the two sites. No structural change has been detected at the transition.