THE ORTHO-II SUPERSTRUCTURE OF YBA2CU3O6.5 - TEMPERATURE BEHAVIOR OXYGEN ORDERING AND T-C

The ortho-II structure of YBa2Cu3O6.5 is investigated in a temperature range between 20 K and 410 K with X-ray diffraction at single crystal s. Structure refinements with the intensities of 160 symmetry independ ent superstructure reflections show that the antiferro-distortive disp lacements of Y and Ba increase at lower temperatures. There is no sign ificant change of the displacements at T-c. In the range between 298 a nd 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 ki netic of oxygen ordering after a heat treatment and the change of the antiferro-distortive displacements of Y and Ba. The relation between t he degree of oxygen order and the superconducting transition temperatu re is established for T-c values between 50 and 53 K. (C) 1998 Elsevie r Science B.V.