Evidence of apical oxygen inNd2CuOydetermined by single-crystal neutron diffraction

The defect structure of oxygenated and reduced ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_{\mathit{y}}$ (y≊4) was investigated by single-crystal neutron diffraction. Structural refinements indicate the presence of interstitial oxygen atoms in the ‘‘apical’’ O(3) position, directly above or below the copper atoms. The occupancy of the apical oxygen site is ∼0.10 (per formula unit) for the oxygenated sample and ∼0.04 for the reduced one. Both the in-plane O(1) and the out-of-plane O(2) oxygen sites were found to be deficient, but their occupancy did not vary, within the error bars, between the two crystals. The difference in oxygen content between the two crystals, as calculated from the structural refinements, is consistent, within the error bars, with the thermogravimetric analysis values reported in the literature.