Structures of incommensurate and commensurate composite crystals NaₓCuO₂ (x = 1.58, 1.6, 1.62)

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van Smaalen, Sander ; Dinnebier, Robert E. ; Sofin, Mikhail ; Jansen, Martin:
Structures of incommensurate and commensurate composite crystals NaₓCuO₂ (x = 1.58, 1.6, 1.62).
In: Acta Crystallographica Section B. Bd. 63 (2007) Heft 1 . - S. 17-25.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S0108768106039462

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Abstract

NaₓCuO₂ (x ≃ 1.6) has been synthesized for different compositions x, resulting in both commensurate and incommensurate composite crystals. The crystal structures are reported for two incommensurate compounds (x = 1.58 and 1.62) determined by Rietveld refinements against X-ray powder diffraction data. The incommensurate compounds and commensurate Na₈Cu₅O10 (x = 1.6) are found to possess similar structures, with valence fluctuations of Cu²⁺/Cu³⁺ as the origin of the modulations of the CuO₂ subsystems; the displacive modulations of Na being defined by the closest Na—-O contacts between the subsystems. A comparison of the structure models obtained from single-crystal X-ray diffraction, synchrotron-radiation X-ray powder diffraction and X-ray powder diffraction with CuKα₁ radiation indicates that single-crystal X-ray diffraction is by far the most accurate method, while powder diffraction with radiation from an X-ray tube provides the least accurate structure model.