Superconducting Tl_2.0Ba_2.0CuO_6+δ: A high resolution neutron powder and single crystal x-ray diffraction investigation

The structure of the orthorhombic form of Tl₂Ba₂CuO_6+δ(T_c=90 K), synthesized in sealed gold tubes using BaO₂, has been refined using high resolution neutron powder diffraction data collected at 4 K. No significant deficiency in metal stoichiometry could be inferred from the crystal structure analysis. The space group (Fmmm) used in this study gives essentially the same results as the alternative (Abma, Amaa) proposed in earlier work. The large amount of disorder that exists in the Tl₂O₂-slab is mirrored in the large thermal parameters for this structural unit compared to those for the CuO₂ sheet. A site at ( built1 4 built1 4 built1 4) is occupied by an amount of oxygen to give overall stoichiometry Tl₂Ba₂CuO_6.10, suggesting that oxygen non-stoichiometry plays an important role in producing high T_c superconductivity in this system. Analysis of single crystal X-ray diffraction data of tetragonal Tl₂Ba₂CuO_{6+ δ} is totally consistent with the neutron powder diffraction results.