DETERMINATION OF THE 2-DIMENSIONAL INCOMMENSURATELY MODULATED STRUCTURE OF MO2S3

The incommensurately modulated structure of molybdenum sesquisulfide M o2S3, with modulation wavevectors q1 = -0.056 (1)a + 0.500 (1)b* + 0. 229 (1)c and q2 = 1/2* + 0.469 (1)b* at room temperature, has been de termined by X-ray diffraction. The lattice parameters of the primitive monoclinic cell are: a = 6.0868 (4), b = 3.2046 (1), c = 8.6233 (5) a ngstrom, beta = 102.443 (6)-degrees [with alpha = 89.995 (5) and gamma = 90.010 (7)-degrees], V = 164.3 angstrom3, Z = 2, mu = 90.4 cm-1 (la mbda = 0. 7107 angstrom), M(r) = 288.1. The symmetry of the structure can be described as consisting of two independent one-dimensional modu lated systems each with a four-dimensional superspace group with the s ame subcell structure P2(1)/m. The q1 system is described with P(P1BAR /1BAR)(alphabetagamma) and consists of two domains (related by m(y)), and the q2 system is described with A(P21/1/m/1BAR)(1/2beta0). The fin al R factor is 0.063 for 10 264 reflections. Alternatively, the struct ure can be described with the triclinic five-dimensional superspace gr oup P1BAR/P2(alphabetagamma)(lambdamunu) giving a final R(F) factor of 0.061 for 13 164 reflections. In the modulated structure the two inde pendent Mo zigzag chains are distorted towards different types of four - and three-metal-atom clustering, with the S atoms squeezed out so as to keep the Mo-S distances as for the undistorted case.