General trends in the structural, electronic and elastic properties of the M₃AlC₂ phases (M = transition metal): A first-principle study

In this paper, the first-principles pseudopotential total energy method is used to predict the structural, electronic and elastic properties of the M ₃AlC₂ (MAX) phases, where M = 3d, 4d, and 5d early transition metals. Specifically, the effects of the valence electron concentrations (VEC) of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W were examined. The lattice constants are a linear function of the atomic diameter of the M element. In general, M d-Al p hybridizations locate just below the Fermi level and are weaker than the M d-C p bonds, which are deeper in energy. The bulk moduli of the ternary carbides are found to be proportional to the bulk moduli of the corresponding binary carbides. Because the M-Al bonds are less stiff than the M-C bonds, the latter are mainly responsible for the high bulk moduli of the M₃AlC₂ phases. The M-Al bonds, on the other hand, play a critical role in decreasing the bulk moduli compared to the binary carbides.