Bonding and classification of nanolayered ternary carbides

We have investigated the elastic properties of nanolayered ${\mathrm{M}}_{2}\mathrm{AC}$, with $\mathrm{M}=\mathrm{Ti}$, Zr, Hf, V, Nb, Ta, Cr, Mo, W and $\mathrm{A}=\mathrm{Al}$, Ga, Ge, Sn, by ab initio calculations. We suggest that ${\mathrm{M}}_{2}\mathrm{AC}$ can be classified into two groups: One where the bulk modulus of the binary MC is conserved and another group where the bulk modulus is decreased. This classification can be understood in terms of coupling between MC and A layers, which is defined by the valence electron population. These results may have implications for the understanding of properties and the performance of this class of solids.