Neutron-diffraction study of ${\mathrm{ErBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{x}}$ in the composition range 6.1≤x≤7.0

Structural properties of homogeneous orthorhombic and tetragonal ${\mathrm{ErBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{x}}$ powder samples (6.1≤x≤7.0), prepared by a careful absorption-desorption procedure, were investigated by elastic neutron scattering. With decreasing oxygen content the distance between Cu(2)-O(2,3) planes and Cu(1)-O(4)-Cu(1) chains increases and the oxygen 2q(0,0,z) apex atoms O(1) move closer to the Cu-O-Cu chains. The copper-oxygen (1) distances do not vary continuously in the z direction but seem to change more rapidly when x<6.5. The change of these distances can be correlated to the suppression of superconductivity in orthorhombic ${\mathrm{ErBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{x}}$. A distinction between the large anisotropic vibrational amplitudes of the Cu(1)-O(4)-Cu(1) broken-chain oxygen atoms and a split-site model for O(4) is not possible on the basis of the present data.