Oxygen configuration and disordering in the superconductor (Hg_0.7Tl_0.3)₂Ba₂(Y _0.8Ca_0.2)Cu₂O_8+δ

The structure of (Hg_0.7TI_0.3)₂Ba₂(Y _0.8Ca_0.2)Cu₂O_8+δ with a T_c of 65 K was determined by neutron powder diffraction. Rietveld refinement based on space group I4/mmm showed a highly disordered atomic arrangement on double HgO_1+δ/2 sheets. Both Hg/TI and O3 atoms on the double sheets had apparently large isotropic atomic displacement parameters, which results from incomplete modelling of their disordered configuration. The occupation factor, g, of the O3 site split into four pieces, was reported to be 0.194 in Hg₂Ba₂YCu₂O_7.55, whereas g(O3) was very near to 1/4 in (Hg_0.7TI_0.3)₂Ba₂(Y _0.8Ca_0.2)Cu₂O_8+δ (δ ≈ 0). Such negligible oxygen deficiency is evidently due to partial substitution of TI³⁺ ions for Hg²⁺ ions. Distances between apical oxygen, O2, and cations surrounding it are discussed in detail from a crystal chemistry point of view. A bond-valence sum of +2.084 for Cu in the present oxide supports the idea that it is underdoped with hole carriers.