Crystal structure of the high-temperature superconductor TI₂Ba₂CaCu₂O₈

Abstract

High-temperature superconductivity has gained renewed attention following recent reports of the onset of superconductivity above 100 K in oxide systems containing copper, alkaline earths and bismuth¹ or thallium^2–4. The superconducting oxide in the Bi–Sr–Ca–Cu–O system has been identified as Bi₂Sr_3–xCa_xCu₂O₈, and its structure comprises pairs of CuO₂ sheets, interleaved by Ca(Sr), alternating with double bismuth-oxide layers^5–7, Hazen et al.⁸ have identified two new superconducting phases, Tl₂Ba₂Ca₂Cu₃O₁₀ (2223) and Tl₂Ba₂CaCu₂O₈ (2212), both with tetragonal or pseudotetragonal structures that are probably related to the structure of Bi₂Sr_3–xCa_xCu₂O₈. Here we present X-ray diffraction data for a single crystal of the 2212 phase, which has an onset temperature of ∼112 K. The crystal structure, based on a body-centred tetragonal cell with a =3.8550(6) Å and c = 29.318(4) Å (space group I4/mmm), is essentially the same as that of Bi₂Sr_3–xCa_xCu₂O₈ and comprises double copper-oxygen sheets alternating with double thallium-oxygen layers. There are Ca²⁺ cations between adjacent Cu–O sheets and Ba²⁺ cations between the Cu–O and Tl–O layers. The copper–oxygen chains present in YBa₂Cu₃O₇ are absent in Tl₂Ba₂CaCu₂O₈.