The calculated formation enthalpies of Fe3W3C (W in the 48f site) and Fe2W4C are negative, and they are of higher stability than other (Fe,W)(6)C phases. The 16d and 32e sites are comfortable for Fe atoms, while the W atom is preferential in the 48f site during the formation process of (Fe,W)(6)C carbides. An exchange of electrons takes place between the Fe/W and the C atom, and also appears between the Fe and W atom. In M-M bonds, besides metallic characters, a valence character between W W bonds exists. Besides the nonmetal C atom with negative charges (obtaining electrons), the partial metal atoms of Fe and W in different M6C-type carbides show an electronegative phenomenon. The carbides W6C, Fe5WC, Fe2W4C and Fe3W3C-II likely have high temperature superconductive characters. The hardness of Fe3W3C (16.5 GPa) is the highest in these phases, and that of Fe2W4C (13.7 GPa) is the second highest. The Debye temperature (theta(D)) of Fe3W3C-I (Fe in the 48f site) is lowest (254 K) in these carbides, and that of Fe3W3C-II suddenly changes to 568 K when W is in the 48f site. The change of the atom in the 48f site intensively influences the properties of M6C (M = Fe/W). (C) 2015 Elsevier B.V. All rights reserved.