Verification of stability and unraveling the electronic and physical properties of bulk and (001)-surfaces of newly synthesized Ti₂ZnX (X = C, N) MAX phases

MAX phase family has been extended by the addition of late transition metals at the A-site with the expectation of diverse functional properties. Here, we present our systematic density functional investigation on the thermodynamic and phonon stabilities, elastic properties, including elastic constants, elastic moduli and elastic anisotropy of newly synthesized Ti₂ZnX (X = C, N) phases in comparison with conventional Ti₂AlX (X = C, N). Due to the smaller size of N as compared to C, the unit cell dimension is reduced when C atoms are replaced by N atoms at the X-site. The Ti₂ZnC and Ti₂ZnN are stable at the equilibrium volume of 110.84 ų and 105.70 ų. The thermodynamic, mechanical and dynamical stabilities are validated by estimating the formation energies, elastic constants and phonon dispersions, respectively. The elastic properties of Ti₂ZnN are less anisotropic as compared to those of Ti₂ZnC. To understand the thin-film characteristics in Ti₂ZnX, the surface properties with (001)-terminated slabs are investigated. Both Ti₂ZnX bulk and (001)-surfaces exhibit metal-like electronic structures. There is a strong covalent bonding between Ti-X and Ti-Zn atoms confirmed by the charge density map and Mulliken population analysis. Additional states are generated at the Fermi level (E_F) due to the unusual d-p states hybridization between Ti and Zn atoms. The anisotropy in chemical bonding is confirmed by the cleavage energy difference between Ti-X and Ti-Zn. Here, Ti(X)-001 and Zn-001 terminations are stable surfaces; however, in terms of chemical potentials, Zn-001 termination is the most favourable in Ti₂ZnX.