X-ray diffraction study of Nd₂CuO₄ single crystals at 20 K

Single crystals of Nd₂CuO₄, a parent compound for the electron-doped high-T_c superconductors, have been studied using X-ray diffraction data collected at 20 K. Structure refinements indicated that the specimen was nonstoichiometric, with the approximate formula Nd_1.89CuO_3.85. Difference Fourier maps showed significant residual peaks around the Cu and Nd atoms. The residual density around the Cu atom had features characteristic of the valence state +2. Part of the nonspherical effects observed around the Nd atom could be described using the Gram-Charlier series expansion of the Gaussian density function, indicating either static or dynamic displacements of this atom, while part of the residual density was attributed to bonding effects. The bonding of the O(1) atom, located in the plane of the Cu atoms, was found to be different from the bonding of the O(2) atom, situated between the Cu-O(1) layers.

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