The ortho-II superstructure of YBa2Cu3O6.5 -: temperature behaviour oxygen ordering and Tc

The ortho-II structure of YBa(2)Cu(3)O(6.5) is investigated in a temperature range between 20 K and 410 K with X-ray diffraction at single crystals. Structure refinements with the intensities of 160 symmetry independent superstructure reflections show that the antiferro-distortive displacements of Y and Ba increase at lower temperatures. There is no significant change of the displacements at T(c). In the range between 298 and 393 K, the ortho-II structure is gradually destroyed. We determine the degree of ortho-II order, the size of the ortho-II domains, the kinetic of oxygen ordering after a heat treatment and the change of the antiferro-distortive displacements of Y and Ba. The relation between the degree of oxygen order and the superconducting transition temperature is established for T(c) values between 50 and 53 K. (C) 1998 Elsevier Science B.V.