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

摘要

NaxCuO2 (x similar or equal to 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 Xray powder diffraction with Cu K alpha(1) radiation indicates that single-crystal X-ray diffraction is by far the most accurate method, while powder diffraction with radiation from an Xray tube provides the least accurate structure model.