Barium displacements in the superconducting ortho-II phase of YBa 2Cu 3O 6.51: An X-ray structure determination

Abstract

From the intensities of superstructure reflections the structure of the ortho-II phase in YBa 2Cu 3O 6.51(5) (T c =55.8 K) was derived by single crystal X-ray diffraction. In the basal plane of the 2 a× b× c supercell the oxygen atoms order in alternating CuOCu and CuCu chains. The correlation lengths of the ortho-II domains are 68 Å, 520 Å, 65 Å in a, b, and c- direction. Due to the oxygen order Ba atoms are displaced in an antiferrodistortive way towards the CuO chains by 0.03 Å. In this way the Coulomb energy of the ortho-II structure is reduced. The antiferrodistortive displacements of the Ba atoms could be important for the superconducting pairing mechanism.