ISSN:
1600-5740
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
The crystal structure of a layered compound Sc2B1.1C3.2, scandium boride carbide (Mr = 140.43), has been re-refined as a commensurate composite crystal using 1795 single-crystal X-ray 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 subsystem 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-dimensional formalism based on the trigonal superspace group P\bar{3}m1(p00)(0p0)0m0. The unit cell and other crystal data are a = b = 3.387 (1), c = 6.703 (2) Å, V = 66.59 (1) Å3, \boldsigma_{1} = (9/7 0 0), \boldsigma_{2} = (0 9/7 0), Z = 1, Dx = 3.501 Mg m−1. Two different three-dimensional sections through the superspace were analyzed, corresponding to two different superstructure models, one with P\bar{3}m1 and the other with P\bar{3}m1. A random distribution of B and C was assumed in the graphite-like layer and 41 structural parameters were introduced. RF/wRF} were 0.0533/0.0482 and 0.0524/0.0476, respectively, for the first and second models. Although the difference between these RF or wRF values was too fine to exclude one of the models definitely, the advantages of using a superspace group were obvious. It not only brought about better convergence of refinement cycles by virtue of fewer parameters, but also gave an insight into the problem of symmetry of the superstructure.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S0108768101006607
