Crystal structure of the superconducting infinite-chain cuprate Ba 2Cu 2.89O 6− y

Abstract

The orthorhombic structure of Ba 2Cu 2.89O 6− y (sp. gr. Pccm, a=13.065(15), b=20.654(21) and c=11.431(8) Å) contains two nonintersecting symmetrically equivalent sets of parallel CuO 2-chains running along [110] and [1̄10] directions. Along the c-axis the Cu–O sheets are separated by identical layers of Ba-atoms. The Cu–O chains are distorted within the xy plane producing zigzag. Three of 14 positions of copper atoms were found to be partly vacant that caused the significant shifts of the neighbouring Ba atoms in the plane. The determined structure of Ba 2Cu 2.89O 6− y is compared to that of the monoclinic Ba 2Cu 3O 6 modification, of another infinite-chain compound Sr 0.73CuO 2, and to one of the substructures of ladder compounds (Sr,Ca) 14Cu 24O 41+ y . The superconductivity in Ba 2Cu 2.89O 6− y is associated with the CuO 2-chains consisting of CuO 4-squares sharing edges. The attention is paid to the coincidence of the temperatures of the superconducting transitions in Ba 2Cu 2.89O 6− y and in ladder compound with the temperature of the sharp magnetic transition in another infinite-chain compound Sr 0.73CuO 2.