First-principles insights into thermo-mechanical, electronic, and optical properties of Hf ₂ AC (A = Cl, Br) MAX phases through A-site halogen tuning

In this study, we conduct a comprehensive first-principles investigation of the mechanical, electronic, optical, and thermo-mechanical properties of Hf₂AC MAX phases, where the A-site halogen is varied between Cl and Br. We use density functional theory to look into how changing the A-site atom affects the structural stability and other functional properties of these layered ternary carbides. Structural analysis reveals that replacing Cl with the larger Br atom increases lattice constants and unit cell volume, aligning with the atomic size trend. Both phases satisfy key stability criteria, including thermodynamic, mechanical, and dynamic stability, confirmed by formation energies, elastic constants, and phonon dispersions. Mechanical property analysis shows that Hf₂BrC has slightly reduced stiffness compared to Hf₂ClC due to weaker Hf-Br bonding, though both maintain ductility and mechanical robustness. Thermo-mechanical parameters, such as Debye temperature, melting point, and thermal conductivity, are influenced by halogen mass and bonding character. Thermo-lattice properties, including heat capacity and the Grüneisen parameter, further support their thermal stability over a broad temperature range. The electronic band structures and density of states show that both compounds act like metals, with Hf d-orbitals at the Fermi level being the most important. Optical properties, derived from the dielectric function, indicate strong activity in the visible and ultraviolet regions. Br substitution causes a red shift in absorption and reflectivity spectra, enhancing plasmonic and photonic behavior. High reflectivity and photoconductivity beyond the plasma frequency highlight their potential in optoelectronic and UV shielding applications. The ability to fine-tune these materials through atomic-level modifications opens new pathways for designing MAX phases with tailored performance for use in coatings, electronics, and energy-related applications.