Superconducting Tl 2.0Ba 2.0CuO 6+δ: A high resolution neutron powder and single crystal x-ray diffraction investigation

Abstract

The structure of the orthorhombic form of Tl 2Ba 2CuO 6+δ( T c=90 K), synthesized in sealed gold tubes using BaO 2, 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 Tl 2O 2-slab is mirrored in the large thermal parameters for this structural unit compared to those for the CuO 2 sheet. A site at ( built1 4 built1 4 built1 4 ) is occupied by an amount of oxygen to give overall stoichiometry Tl 2Ba 2CuO 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 Tl 2Ba 2CuO 6+ δ is totally consistent with the neutron powder diffraction results.