First-Principles Study of Vacancies in Ti(3)SiC(2) and Ti(3)AlC(2)

MAX phase materials have attracted increased attention due to their unique combination of ceramic and metallic properties. In this study, the properties of vacancies in Ti(3)AlC(2) and Ti(3)SiC(2), which are two of the most widely studied MAX phases, were investigated using first-principles calculations. Our calculations indicate that the stabilities of vacancies in Ti(3)SiC(2) and Ti(3)AlC(2) differ greatly from those previously reported for Cr(2)AlC. The order of the formation energies of vacancies is V(Ti(a)) > V(Ti(b)) > V(C) > V(A) for both Ti(3)SiC(2) and Ti(3)AlC(2). Although the diffusion barriers for Ti(3)SiC(2) and Ti(3)AlC(2) are similar (~0.95 eV), the properties of their vacancies are significantly different. Our results show that the vacancy–vacancy interaction is attractive in Ti(3)AlC(2) but repulsive in Ti(3)SiC(2). The introduction of V(Ti) and V(C) vacancies results in the lattice constant c along the [0001] direction increasing for both Ti(3)SiC(2) and Ti(3)AlC(2). In contrast, the lattice constant c decreases significantly when V(A) are introduced. The different effect of V(A) on the lattice constants is explained by enhanced interactions of nearby Ti layers.