Superconducting Tl₂Ba₂CuO₆: The orthorhombic form

The structure of an orthorhombic variant of Tl₂Ba₂CuO₆ (Z = 4), with a superconducting onset at 90 K, has been studied using neutron powder diffractometry at 12 K (a = 5.4834(3), b = 5.4586(3), c = 23.198(1) Å), 60 K (a = 5.4834(3), b = 5.4585(3), c = 23.199(1)Å), and 293 K (a = 5.4967(3), b = 5.4651(3), c = 23.246(1) Å). The distortion from I4 mmm symmetry, reported in single-crystal X-ray investigations, to Abma is manifested in the TIO layer. At the temperatures studied, the coordination of TI to oxygen is (2 + 1 + 2) square pyramidal rather than octahedral. At lower temperatures, distortion decreases but does not disappear. Despite the orthorhombic symmetry the CuO₂-layers remain flat.