Crystal structure of Y_0.9Ba_2.1Cu₃O₆, a compound related to the high-T_c superconductor YBa₂Cu₃O₇

Abstract

The crystal structure of the high-temperature superconductor YBa₂Cu₃O₇ is derived from the ideal perovskite (YBa₂Cu₃O₉) structure by the presence of ordered vacancies in the oxygen sublattice. Owing to the ordering of these vacancies, the coordination polyhedra around the copper cations are either pyramids or slightly distorted squares^1–5; the one-dimensional character and the orthorhombic symmetry of the structure are due to the existence of the squares. We have synthesized single crystals of the further-reduced compound Y_0.9Ba_2.1Cu₃O₆, and we report here a structural determination using single-crystal X-ray diffraction data. Y_0.9Ba_2.1Cu₃O₆ has a tetragonal oxygen-deficient perovskite structure related to that of YBa₂Cu₃O₇. The additional oxygen vacancies transform the CuO₄ squares into O–Cu–O linear bonds, while the pyramids are not affected. As Y_0.9Ba_2.1Cu₃O₆ does not exhibit any superconducting transition down to 4.2 K, we conclude that the chains of CuO₄ squares are essential for the superconductivity mechanism.