Structure of the 100K Superconductor Ba sub 2 YCu sub 3 O sub 7 between 5 & 300K by neutron powder diffraction.

Recently Siegrist et al. [5] proposed a structure for a high T sub c superconductor Ba sub 2 YCu sub 3 O sub (9-del) based on an orthorhombic (a=a sub p, b=a sub p, c=3a sub p) perovskite- like model containing Ba and Y cations ordered over the A-sites of the ABO sub 3 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 5K and 300K, we have refined the structure of a pure and well characterized powder sample with onset of superconductivity at 100K. 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.