DFT insights into the role of A elements on the phase stability, crystal structure and properties of recently discovered M ₃ AC ₂ and Sc ₂ AC

Density functional theory (DFT) was utilized to obtain theoretical insights into the recently discovered M₃AC₂ (M = Zr, Hf; A = In, Pb, Cd, Sb) and Sc₂AC (A = Al, Ga, In), with calculations and comparisons of phase stabilities, crystal structures, electronic structures, chemical bonding, elastic and thermal properties, static exfoliation energy, as well as cleavage energy and theoretical tensile strength. After verifying the intrinsic, mechanical, and thermodynamic stability, their crystal structures were accurately predicted by DFT. According to the calculated "bond stiffness" and associated criteria, it is predicted that Zr₃SbC₂ and Sc₂AC (A = Al, Ga, In) are intrinsically brittle, in contrast to the other and typical MAX phases with high damage tolerance and fracture toughness. Furthermore, the valence electron concentration (VEC) of A elements significantly affects the stiffness of M-A bonds by modulating electron transfer between M and A, which impacts the electronic structure, compressibility, and elastic and thermal properties of the MAX phases. Considering contributions from phonons and electrons, the linear thermal expansion coefficients [(5-15) × 10^-6 K^-1, 300-1100 K] and heat capacities as functions of temperature were predicted. Finally, the influence of the VEC of component A on the mechanical properties and specific functionalities of MAX phases, in the form of nanostructures, was discussed, and the feasibility of exfoliating the researched MAX phases to form two-dimensional systems was evaluated.