Published online by Cambridge University Press: 28 February 2011
The crystal structure of several RBa2Cu3-yO6+z (R=Y,Ho) crystals are investigated by neutron diffraction using an exact solution for the twinned orthorhombic samples. For an oxygen concentration >0.35 in the chains, the structure is orthorhombic due to an exclusive occupancy of the 0(4) site. A systematic and variable copper deficiency is observed on the Cu(l) site leading to a correlated defect on the chain oxygen O(4) and on the bridging oxygen O(l). Due to the presence of these defects, a transverse disorder is observed for the highest oxygen content with a residual occupancy of the forbidden O(5) site.