DFT insights into Nb-based 211 MAX phase carbides: Nb2AC (A = Ga,Ge, Tl,Zn, P,In, and Cd)

In this study, we performed the first-principles calculations to study the 211 MAX phase carbides: Nb₂AC (A = Ga, Ge, Tl, Zn, P, In, Cd, and Al). The structural characteristics are in good agreement with those of the prior studies. The mechanical behavior has been explored by calculating the stiffness constants, elastic moduli, and Vickers hardness. The stiffness constants and phonon dispersion curves were used to check the structural stability of the selected compounds. 2D and 3D plotting of elastic moduli and calculated anisotropy indices disclosed the anisotropy of the elastic properties. We utilized the Mulliken atomic and bond overlap population to explain the mixture of ionic and covalent bonding among these carbides. The metallic behavior has been confirmed by calculating the band structure and density of states (DOS). Partial DOS was also used to discuss the bonding nature and strength among the different states. The optical properties of these phases have also been computed and analyzed to reveal possible relevance in diverse fields. The Debye temperature (Θ_D), Grüneisen parameter (γ), melting temperature (T_m), and minimum thermal conductivity (K_min) were studied to bring out their possible relevance in high-temperature technology. The outcomes of this research indicate that the titled carbides are suitable for use as solar radiation-protecting coating and thermal barrier coating (TBC) materials.

Comparison of (a) stiffness constants and (b) elastic moduli of Nb₂AC (A = Ga, Ge, Tl, Zn, P, In, Cd, and Al) MAX phases.