Evidence of apical oxygen in Nd2CuOy determined by single-crystal neutron diffraction.

The defect structure of oxygenated and reduced Nd[sub 2]CuO[sub [ital y]] ([ital y][approx]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 [similar to]0.10 (per formula unit) for the oxygenated sample and [similar to]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.