Simulation of various possible cementite structures

The simulation of the cementite structure has been performed without resorting to any preliminary assumptions. The spheres simulating the Fe and C atoms are closely packed in accordance with the rules of pair interpenetration. The maximum depth of the interpenetration in the Fe-C pair was accepted to be intermediate between the maximum depths of the penetration in the Fe-Fe and C-C pairs in ferrite and diamond, respectively. Under such conditions, six iron atoms penetrate into a carbon atom. The carbon atom turns out to be located inside octahedra, which can be of various shapes, while the centers of iron atoms occupy the vertices of these octahedra. The translation of the octahedra leads to the formation of orthorhombic cementite structures with identical or close parameters, but with different relative positions of carbon and iron atoms.