Cr23C6 crystal structure

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[1] as part of the chromium-carbon binary phase diagram. Over 85 known compounds adopt this structure type,[2] which can be described as a NaCl-like packing of chromium cubes and cuboctahedra.[3]

## Contents

* 1 Structure
* 2 Materials
* 3 References
* 4 External links

## Structure

The space group of this structure is called Fm3m (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 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.

## Materials

Examples of compounds that form in this structure type include Cr23C6, Mn23C6, and many ternary intermetallic carbides and borides.[4] A few phases of ternary silicides, germanides, and phosphides are also known to exist.[5][6] In going from the binary to ternary systems, some of the transition metal atoms are substituted by a third element, which can be an alkali metal, alkaline earth metal, rare-earth element, main group element, or another transition metal. This leads to an empirical formula of the form A23−xBxC6. Materials of this kind continue to be studied for potentially interesting magnetic and physical properties.[7][8]

## References

1. ↑ Westgren, A. Crystal structure and composition of cubic chromium carbide. Jernkontorets Ann. 1933, 117, 501.
2. ↑ Villars, P. Pearson's Handbook of Crystallographic Data for Intermetallic Phases, vol. 1; ASM International: Materials Park, 1997.
3. ↑ Bowman, A. L.; Arnold, G. P.; Storms, E. K.; Nereson, N. G. (1972-10-15). "The crystal structure of Cr23C6". Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry (International Union of Crystallography (IUCr)) 28 (10): 3102–3103. doi:10.1107/s0567740872007526. ISSN 0567-7408.
4. ↑ Stadelmaier, H. H. Developments in the Structural Chemistry of Alloy Phases; Plenum Press: New York, 1969; p 141.
5. ↑ Gribanov, A.V.; Seropegin, Yu.D.; Bodak, O.I. (1994). "Crystal structure of the compounds Ce3Pd20Ge6 and Ce3Pd20Si6". Journal of Alloys and Compounds (Elsevier BV) 204 (1-2): L9–L11. doi:10.1016/0925-8388(94)90057-4. ISSN 0925-8388.
6. ↑ Keimes, V.; Mewis, A. (1992). "Mg3Ni20P6 und Mn3Ni20P6 \- Zwei neue Phosphide mit Cr23C6-Struktur" (in de). Zeitschrift für anorganische und allgemeine Chemie (Wiley) 618 (12): 35–38. doi:10.1002/zaac.19926180107. ISSN 0044-2313.
7. ↑ Eriksson, T.; Vennström, M.; Ronneteg, S.; Andersson, Y.; Nordblad, P. (2007). "Complex magnetic properties of Mn3Ni20P6 and ferromagnetic structure of the new isostructural compound Mn3Pd20P6". Journal of Magnetism and Magnetic Materials (Elsevier BV) 308 (2): 203–209. doi:10.1016/j.jmmm.2006.05.016. ISSN 0304-8853.
8. ↑ Veremchuk, I.; Gumeniuk, R.; Prots, Yu.; Schnelle, W.; Burkhardt, U.; Rosner, H.; Kuz'ma, Yu.; Leithe-Jasper, A. (2009). "Crystallographic and physical properties of RE2−xNi21B6 (RE=Er, Yb and Lu)". Solid State Sciences (Elsevier BV) 11 (2): 507–512. doi:10.1016/j.solidstatesciences.2008.09.004. ISSN 1293-2558.

## External links

* International Crystal Structure Database

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