SmC6 crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Sm2+ is bonded to twelve equivalent C+0.33- atoms to form face-sharing SmC12 cuboctahedra. All Sm–C bond lengths are 2.75 Å. C+0.33- is bonded in a 3-coordinate geometry to two equivalent Sm2+ and three equivalent C+0.33- atoms. There is two shorter (1.44 Å) and one longer (1.45 Å) C–C bond length.