First-principles study on stability and electronic properties of MC and Mn+1 ACn phases

Investigation of the stability and electronic properties of a series of MC compounds and classic M AX phases, M(n+1)AC(n) (M = Sc, Ti, V, Cr, and Mn; A = Al, Si, P, and S; n = 1, 2, and 3), contributes to finding the intrinsic mechanism of the stability of M(n+1)AC(n) and to the design of new M(n+1)AC(n) phases. First-principles calculations show that the formation enthalpy of both MC and M(n+1)AC(n) is directly correlated with the charge transfer from M-3d to C-2s and 2p orbitals. Correspondingly, the early transition metals with high electron donation ability are able to form stable MC phases. Among the various MC phases, MC is found to be electron-deficient, which is thus favorable to react with electron-abundant M A to form M(n+1)AC(n). Therefore, M2AlC and M2SiC can be more readily separated into two-dimensional M2C structures, compared to M2PC and M2SC.