Crystal chemistry of HgBa2Can-1CunO2n+2+delta (n=1, 2, 3, 4) superconductors

Constant wavelength neutron powder diffraction data at room temperature were used to refine the crystal structures of HgBa2Can-1CunO2n+2+delta (n = 1, 2, 3, 4) superconductors, which were synthesized by solid-state reaction of the component oxides. Samples annealed both in oxygen and argon atmospheres were examined. Rietveld refinements converged to values of R(wp) 8-10% and chi(2) = 1.1-1.7 using a tetragonal cell with P4/mmm symmetry. The HgBa2CuO4+delta, HgBa2CaCu2O6+delta, and HgBa2Ca2Cu3O8+delta (n = 1, 2, and 3) samples were nearly single phase, whereas the HgBa2Ca3Cu4O10+delta (n = 4) sample was primarily a mixture of the a = 3 and 4 phases. For increasing n, the oxygen-annealed samples exhibit T-c (diamagnetic onsets) of 94, 123, 134, and 124 K, respectively, and a contraction of the CuO2 sheets (attice parameter a). The highest T-c sample, HgBa2Ca2Cu3O8+delta, has the shortest apical Cu-O bond. Consistently, the atomic displacement parameter for Hg is large. For refinements in which the Hg atomic displacement parameter is fixed at a more reasonable value, occupancies of the Hg site are significantly less than full for the n > 1 samples, suggesting that the Hg site is deficient and/or possibly occupied by atoms with smaller scattering lengths, e.g., Cu. The oxygen site within the Hg layer is partially occupied, and its occupation increases with increasing a. For the argon-annealed samples, the cell volume increases slightly due to the isotropic increase of both the cell parameters and T-c decreases. For HgBa2CaCu2O6+delta, the occupancy of the oxygen site within the Hg layer decreases upon annealing in argon, yet the T-c increased, implying that the initial carrier concentration is in the overdoped regime. (C) 1996 Academic Press, Inc.