Structure refinement of the layered composite crystal SC2B1.1C3.2 in a five-dimensional formalism

The crystal structure of a layered compound Sc2B1.1C3.2, scandium boride ca rbide (M-r = 140.43), has been re-refined as a commensurate composite cryst al using 1795 single-crystal Xray diffraction intensities with I > 2 sigma (I) collected by Shi, Leithe-Jasper, Bourgeois, Bando & Tanaka [(1999), J. Solid State Chem. 148, 442-449]. The crystal is composed of two layered sub system structures, i.e. Sc-C-Sc sandwiches and graphite-like layers of the composition B1/3C2/3. The structure refinement was performed in a five-dime nsional formalism based on the trigonal superspace group P(3) over bar ml ( p00)(0p0)0m0. The unit cell and other crystal data are a = b = 3.387 (1), c = 6.703 (2) Angstrom. V = 66.59 (1) (3), sigma (1) = (9/7 0 0), sigma (2) = (0 9/7 0), Z = 1, D-x = 3.501 Mg m(-3). Two different three-dimensional s ections through the superspace were analyzed, corresponding to two differen t superstructure models, one with P(3) over bar ml and the other with P3ml. A random distribution of B and C was assumed in the graphite-like laver an d 41 structural parameters were introduced. R-F/wR(F) were 0.0533/0.0482 an d 0.0524/0.0476, respectively, for the first and second models. Although th e difference between these R-F or wR(F) values was too fine to exclude one of the models definitely, the advantages of using a superspace group were o bvious. It not only brought about better convergence of refinement cycles b y virtue of fewer parameters, but also gave an insight into the problem of symmetry of the superstructure.