Ab initio study of polymorphism in layered ternary carbide M4AlC3 (M = V, Nb and Ta)

The mechanical and thermodynamic stabilities of M4AlC3 (M = V, Nb and Ta) and Ti4AlN3 polymorphs were investigated by means of the first-principles pseudopotential total energy method. All compounds satisfied the Born criteria for mechanical stability, but had different thermodynamic stabilities. Only Ta4AlC3 showed a possible polymorphic phase transformation driven by thermodynamic competition. The present theoretical results support the polymorphism of Ta4AlC3 in experimental synthesis and explain the underlying thermodynamic mechanism. Polymorphism was excluded from V4AlC3, Nb4AlC3 and Ti4AlN3.