[en] The formation, crystal structure and magnetic properties of Sm₂Fe_17-xCo_xC_y were studied. The substitution of Co for Fe helps to stabilize the rhombohedral Th₂Zn₁₇-type structure (2:17) and eliminate α-Fe formation for carbon concentrations y ^≤ 1.5. From the mixing enthalpy calculations based on Miedema's semi-empirical model and Wigner-Seitz cell analysis, it is found that the role of Co in stabilizing the 2:17 structure is related to its strong preference for the 18h site, which has a maximum number of rare-earth near neighbours in the structure. With increasing Co content, x, the lattice constants decrease while the Curie temperature rapidly increases. The typical compound Sm₂Fe₁₂Co₅C_1.25 has a room temperature saturation magnetization μ₀M_s = 1.36 T, a magnetic anisotropy field μ₀H_k = 7.1 T, a Curie temperature T_C = 874 K and a possible maximum operating temperature T_m higher than 473 K