First-principles investigation of structural, mechanical, and optoelectronic properties of Hf2AX (A═Al, Si and X═C, N) MAX phases

In this work, we have employed the first-principles calculations to investigate the phase stability and mechanical and optoelectronic characteristics of Hf(2)AX (A=Al, Si and X=C, N) MAX phases. Phase stabilities of Hf(2)AX compounds have been evaluated by the formation enthalpy computations and phonon dispersion curves, which indicate that all studied compounds are structurally and dynamically stable. The mechanical stability has been determined by elastic stiffness constants, which confirms that the studied MAX phases are mechanically stable. The computed results for Pugh's and Poisson ratios indicate the brittle nature of Hf(2)AX MAX phases. It is interesting to note that Si-based MAX phases possess high values of B/G, indicating that they are harder than Al-based compounds. The obtained band structures and partial density of states indicate the metallic character of all Hf(2)AX compounds. The strong hybridization of d-orbitals of Hf with p-orbitals of N and the comparatively weaker hybridization of p-orbitals (Hf) with Al/Si p-orbitals is observed. The presence of pseudogaps near the Fermi energy level emerges due to the orbital hybridization involving Hf, Al/Si, and C/N atoms. The analysis of charge density difference maps reveals the presence of a strong covalent bond between the Hf and C/N atoms, whereas a relatively weaker covalent bond is seen between the Hf and Al/Si atoms. Furthermore, numerous optical characteristics have been investigated to account for the behavior of the Hf(2)AX compounds to those of impinging electromagnetic rays. The highest absorptivity is noticed within the energy range of 7.5-12.5 eV. The optical spectra in the range of 1.7 eV (IR) to 9 eV ultraviolet (UV) have been observed for the investigated MAX phases, predicting their suitability as proficient energy absorbers within the UV region. The Intriguing properties of Hf(2)AX compounds are anticipated to be appropriate materials for a variety of applications.