Neutron Diffraction Study on Orthorhombic YBa₂Cu₃O6.74 and Tetragonal YBa₂Cu₃O6.05

The crystal structures of high T c (∼90 K) orthorhombic YBa 2 Cu 3 O 6.74 and moderate T c (∼50 K tetragonal YBa 2 Cu 3 O 6.05 have been studied by the conventional neutron powder diffraction technique. Single phase powder samples were used. In the orthorhombic phase, the occupancies of the O 1 (0 1/2 0) and the O 4 sites (1/2 0 z), z=0.380(2) are found to be respectively 0.91(3) and 0.92(2). In the tetragonal phase, the occupancies of the O 1 (0 1/2 0) and the O 3 sites (0 1/2 z), z=0.383(2) are 0.20(4) and 0.91(2). Some disordering of Ba and Y atoms is suggested in the tetragonal phase.