Cr23C6 crystal structure
Image:Cr23C6 [structure type with colored polyhedra.jpg|thumb|right|210px|The Cr23C6 structure represented as polyhedra of atoms. The blue surfaces outline cuboctahedra of chromium atoms, whereas the red surfaces outline chromium cubes capped by carbon atoms. The darker blue spheres represent chromium atoms outside of the polyhedra.]
Cr23C6 is the prototypical compound of a common crystal structure, discovered in 1933 as part of the chromium-carbon binary phase diagram. Over 85 known compounds adopt this structure type, which can be described as a NaCl-like packing of chromium cubes and cuboctahedra.
Structure
The space group of this structure is called Fmm or "225". 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 cuboctahedron, 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.