Synthesis, structure and superconductivity of Hg0.75Mo0.25Ba2CuO4+delta

Good quality samples of Hg0.75Mo0.25Ba2CuO4+delta were synthesised in a clo sed-ampoule system. The oxygen stoichiometry and T-c, were varied by anneal ing the samples in reducing and oxidising atmospheres. The relevant structu ral features were determined by powder neutron diffraction data, electron d iffraction, and EDS. The additional oxygen, delta, is incorporated onto the Hg/Mo layer to provide four oxygen neighbours to each Mo. For the as-prepa red sample, delta was found to be 0.59, corresponding to about an extra oxy gen per every other Hg/Mo mesh. As shown by the structural refinement, thes e atoms do not occupy the (1/2, 1/2, 0) position, but are shifted along the diagonal at about (0.37, 0.37, 0) position. The Mo cations are displaced f rom the (000) position along the x-axis at (0.13, 0, 0). These shifts give rise to somewhat distorted oxygen tetrahedra surrounding the Mo cations. No superstructures were detected in any direction by election diffraction, Th e as-prepared samples were found to be overdoped, with T-c = 12 K, After an nealing them in Ar atmosphere delta decreased from 0.59 +/- 0.02 to 0.51 +/ - 0.02. The structural refinement of the sample with delta = 0.51, which ex hibited a ir, of 40 K (with about 20% superconducting volume), showed that the annealing did not induce any major structural changes, except for the M o cations which moved toward the center of the oxygen tetrahedra, This feat ure was taken as indication that a charge transfer occurs during the anneal ing. (C) 1999 Elsevier Science B.V. All rights reserved.