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Cr23C6 crystal structure
Cr23C6 is the prototypical compound of a common crystal structure, discovered in 1933〔Westgren, A. Crystal structure and composition of cubic chromium carbide. ''Jernkontorets Ann.'' 1933, ''117,'' 501.〕 as part of the chromium-carbon binary phase diagram. Over 85 known compounds adopt this structure type,〔Villars, P. ''Pearson's Handbook of Crystallographic Data for Intermetallic Phases, vol. 1; ASM International: Materials Park, 1997.〕 which can be described as a NaCl-like packing of chromium cubes and cuboctahedra.〔Bowman, A. L.; Arnold G. P.; Storms, E. K.; Nereson, N. G. The crystal structure of Cr23C6. ''Acta Cryst.'' 1972, ''B28,'' 3102.〕
==Structure==
The space group of this structure is called F m 3̅ m (in Hermann-Mauguin notation) or "225" (in the International Tables for Crystallography). It belongs to a cubic crystal system, with Pearson symbol cF116.
The shortest interatomic distances are between carbon and chromium atoms, which is expected on the basis of atomic size. The carbon atoms are in positions that cap each face of the chromium cubes and their coordination environment can be thought of as distorted square antiprisms formed from chromium atoms of both the cubes and the cuboctahedra.
The closest Cr-Cr contacts are between members of a cuboctahdron, and the third closest are between members of a cube. The members of the cube, however, are closer to the 8 chromium atoms in the unit cell that are not part of either polyhedron. The coordination environment of these other atoms can be thought of as distorted Friauf polyhedra composed of chromium atoms, if next-nearest neighbors are included.
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