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The ground state physical properties of the newly synthesized 312 MAX compound, Hf 3 AlC 2 have been investigated using the first-principles density functional theory (DFT). The optimized unit cell parameters show good agreement with the experimental values. The calculated elastic constants and phonon dispersion confirm the mechanical and dynamical stabilities of this new compound. High bulk modulus, combined with low shear resistance and low Vickers hardness, indicates good machinability of Hf 3 AlC 2 , as expected for a metallic compound. On the other hand, significant stiffness due to large Young's modulus as well as the brittle nature according to the calculated Pugh's and Poison's ratios and Cauchy pressure are comparable to that of a ceramic. The present calculations show that Hf 3 AlC 2 is elastically and optically anisotropic. The chemical bonding in Hf 3 AlC 2 consists of a mixture of metallic, covalent and ionic contributions. The calculated Fermi surface contains quasi-twodimensional topology, which indicates possible superconductivity of Hf 3 AlC 2 . The new phase Hf 3 AlC 2 may also be a promising thermal barrier coating (TBC) material. The calculated enthalpy and entropy are found to increase with temperature above 100 K though a decrease is observed for the free energy.

The effects of M atomic species mixing on the structural, elastic, electronic, thermodynamic and charge transport properties of newly synthesized MAX phase (Zr 1-x Ti x ) 2 AlC (0 ≤ x ≤ 1) solid solutions have been studied for the first time by means of density functional theory (DFT) based first principles calculations. The lattice constants in good accord with the experimental results, are found to decrease with Ti content. The elastic constants, C ij , and the other polycrystalline elastic moduli have been calculated. The elastic constants satisfy the mechanical stability conditions of these solid solutions. The constants C 11 , C 33 and C 44 are found to increase with Ti contents up to x = 0.67, thereafter these decrease slightly. A reverse trend is followed by C 12 and C 13 . The elastic moduli are also found to increase up to x = 0.67, beyond which these moduli go down slightly. Pugh"s ratio and Poisson"s ratio both confirm the brittleness of (Zr 1-x Ti x ) 2 AlC. Different anisotropy factors revealed the anisotropic character of these solid solutions. A non-vanishing value of the electronic energy density of states (EDOS) at the Fermi level suggests that (Zr 1-x Ti x ) 2 AlC are metallic in nature. A mixture of covalent, ionic and metallic bonding has been indicated from the electronic structure with dominant covalent bonding due to hybridization of Zr-4d states and C-2p states. The variation of elastic stiffness and elastic parameters with x is seen to be correlated with partial DOS (PDOS) and charge density distribution. The calculated Debye temperature and minimum thermal conductivity are found to increase with Ti contents, while melting temperature is the highest for x = 0.67. The solid solution with x = 0.67 shows improved mechanical and thermal properties compared to that of the two end members Zr 2 AlC and Ti 2 AlC. The study of charge transport properties of (Zr 1-x Ti x ) 2 AlC reveals the metallic nature with saturated resistivity. The maximum power factor (S 2 /=11.1×10 10 Wm -1 K -2 s -1 ) is obtained at 400 K for (Zr 1-x Ti x ) 2 AlC.

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In the present study, the structural and hitherto uninvestigated mechanical (elastic stiffness constants, machinability index, Cauchy pressure, anisotropy indices, brittleness/ductility, Poisson's ratio), electronic, optical, and thermodynamic properties of novel boron-rich compounds B 6 X (X = S, Se) have been explored using density functional theory. The estimated structural lattice parameters were consistent with the prior report. The mechanical and dynamical stability of these compounds have been established theoretically. The materials are brittle in nature and elastically anisotropic. The value of fracture toughness, K IC for the B 6 S and B 6 Se are ~ 2.07 MPam 0.5 , evaluating the resistance to limit the crack propagation inside the materials. Both B 6 S and B 6 Se compounds possess high hardness values in the range 31-35 GPa, and have the potential to be prominent members of the class of hard compounds. Strong covalent bonding and sharp peak at low energy below the Fermi level confirmed by partial density of states (PDOS) resulted in the high hardness. The profile of band structure, as well as DOS, assesses the indirect semiconducting nature of the titled compounds. The comparatively high value of Debye temperature (Θ D ), minimum thermal conductivity (K min ), lattice thermal conductivity (k ph ), low thermal expansion coefficient, and low density suggest that both boron-rich chalcogenides might be used as thermal management materials. Large absorption capacities in the mid ultraviolet region (3.2-15 eV) of the studied materials and low reflectivity (~16 %) are significantly noted. Such favorable features give promise to the compounds under investigation to be used in UV surface-disinfection devices as well as medical sterilizer equipment applications. Excellent correlations are found among all the studied physical properties of these compounds.