Crystal chemistry of HgBa{sub 2}Ca{sub n-1}O{sub 2n+2+{delta}} (n = 1,2,3,4) superconductors
- Oak Ridge National Laboratory, TN (United States)
Constant wavelength neutron powder diffraction data at room temperature were used to refine the crystal structures of HgBa{sub 2}Ca{sub n-1}Cu{sub n}O{sub 2n+2+{sigma}}(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{sub wp} = 8-10% and x{sup 2} = 1.1-1.7 using a tetragonal cell with P4/mmm symmetry. The HgBa{sub 2}CuO{sub 4+{sigma}}, and HgBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+{sigma}} (n=1,2 and 3) samples were nearly single phase, whereas the HgBa{sub 2}Ca{sub 3}Cu{sub 4}O{sub 10+{sigma}}(n=4) sample was primarily a mixture of the n=3 and 4 phases. For increasing n, the oxygen-annealed samples exhibit T{sub c} (diamagnetic onsets) of 94, 123, 134, and 124 K, respectively, and a contraction of the CuO{sub 2} sheets (lattice parameter{alpha}). The highest T{sub c} sample, HgBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+{sigma}}, 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 increased 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{sub c} decreases. For HgBa{sub 2}CaCu{sub 2}O{sub 6+{sigma}}, the occupancy of the oxygen site within the Hg layers decreases upon annealing in argon, yet the T{sub c} increased, implying that the initial carrier concentration is in the overdoped regime.
- DOE Contract Number:
- AC05-84OR21400
- OSTI ID:
- 263737
- Journal Information:
- Journal of Solid State Chemistry, Vol. 122, Issue 1; Other Information: PBD: 15 Feb 1996
- Country of Publication:
- United States
- Language:
- English
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