Crystal structure of the superconducting infinite-chain cuprate Ba2Cu2.89O6-y

The orthorhombic structure of Ba2Cu2.89O6-y (sp. gr. Pccm, a = 13.065(15), b = 20.654(21) and c = 11.431(8) Angstrom) contains two nonintersecting sym metrically equivalent sets of parallel CuO2-chains running along [110] and [(1) over bar 10] directions. Along the c-axis the Cu-O sheets are separate d 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 foun d to be partly vacant that caused the significant shifts of the neighbourin g Ba atoms in the plane. The determined structure of Ba2Cu2.89O6-y. is comp ared to that of the monoclinic Ba2Cu3O6 modification, of another infinite-c hain compound Sr0.73CuO2, and to one of the substructures of ladder compoun ds (Sr,Ca)(14)Cu24O41+y. The superconductivity in Ba2Cu2.89O6-y is associat ed with the CuO2-chains consisting of CuO4-squares sharing edges, The atten tion is paid to the coincidence of the temperatures of the superconducting transitions in Ba2Cu2.89O6-y and in ladder compound with the temperature of the sharp magnetic transition in another infinite-chain compound Sr0.73CuO 2. (C) 1999 Published by Elsevier Science B.V. All rights reserved.