Determination of the incommensurately modulated structure of Mo8O23by means of single-crystal X-ray diffraction

Abstract

Mo₈O₂₃ is incommensurately modulated between the temperatures 285 K and 360 K. In this article results are presented on the determination of the structure of this compound at 298 K. The monoclinic cell parameters are determined to be a=13.382(1) AA, b=4.0519(6) AA, c=16.877(2) AA and beta =106.197(3) degrees . The space group of the average structure is found to be P2/c, in accordance with previous studies. The observed satellite reflections can be indexed using a single modulation wavevector, which has components (0.188, 0.5, 0.133). The superspace group characterising the symmetry of the incommensurately modulated structure is B₁₁^P2C/. Refinement on 6270 main reflections and 1940 satellites, with I>2.5 sigma (I), gave convergence to a weighted R-factor of 0.029, with partial R-factors of 0.024 for main reflections and 0.095 for satellite reflections. The modulation is interpreted as a combination of rotations and distortions of the MoO₆ octahedra.