Crystal chemistry of HGBa₂Ca_n-1Cu_nO_2n+2+δ (n = 1, 2, 3, 4) superconductors 1

Constant wavelength neutron powder diffraction data at room temperature were used to refine the crystal structures of HgBa₂Ca_n-1Cu_nO_2n+2+δ (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 X² = 1.1-1.7 using a tetragonal cell with P4/mmm symmetry. The HgBa₂CuO_4+δ, HgBa₂CaCu₂O_6+δ, and HgBa₂Ca₂Cu₃O_8+δ (n = 1, 2, and 3) samples were nearly single phase, whereas the HgBa₂Ca₃Cu₄O_10+δ (n = 4) sample was primarily a mixture of the n = 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 (lattice parameter a). The highest T_c sample, HgBa₂Ca₂Cu₃O_8+δ, 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 n. 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+δ, 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.