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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.

We report the first principles study of structural, elastic, electronic, optical and thermoelectric properties of newly synthesized K 2 Cu 2 GeS 4 . The structural parameters are found to be in good agreement with experimental results. The single crystal elastic constants (C ij ) are calculated and K 2 Cu 2 GeS 4 is found to be mechanical stable. The analysis of polycrystalline elastic constants reveals that the compound is expected to be soft in nature. The values of Pugh and Poisson ratios suggested that the compound lies in the border line of ductile/brittle behavior. The chemical bonding is primarily ionic, the inter-atomic forces are central in nature and the compound is mechanically anisotropic. The computed electronic band profile shows semiconducting characteristics and the estimated band gap is strongly dependent on the functional used representing the exchange correlations. The nature of chemical bonding is explained using electronic charge density mapping. Important optical constants such as dielectric constants, refractive index, absorption coefficient, photoconductivity, reflectivity and loss function are calculated and discussed in detail. Optical conductivity is found to be in good qualitative agreement with the results of band structure calculations. The Seebeck coefficients are positive for the entire temperature range used in this study, suggesting the presence of p-type charge carriers. We have obtained large Seebeck coefficent, 681 V/K at 100 K and 286 V/K at 300 K. At room temperature, the electrical conductivity and electronic thermal conductivity are 1.83×10 18 ms) -1 and 0.5×10 14 W/mK.s, respectively. The dimensionless figure of merit of K 2 Cu 2 GeS 4 is evaluated as ~1.0 at 300 K. This suggests that K 2 Cu 2 GeS 4 is a potential candidate for thermoelectric applications.

We have studied the physical properties of M 2 InC (M = Zr, Hf and Ta) MAX phases ternary carbides using density functional theory (DFT) methodology. The structural, elastic and electronic properties are revisited (and found to be in good agreement with recently reported results). The charge density distribution, Fermi surface features, Vickers hardness, dynamical stability, thermodynamics and optical properties have been investigated for the first time. The calculated single crystal elastic constants and phonon dispersion curves endorse the mechanical and dynamical stability of all the compounds under study. The calculated single crystal elastic constants C ij and polycrystalline elastic constants are found to increase with increasing atomic number of M species (M = Zr, Hf and Ta). The values of Pugh ratio and Poisson"s ratio revealed the brittleness of the compounds under study associated with strong directional covalent bond with a mixture of ionic contribution. Overlapping of conduction band and valence band at Fermi level notify the metallic nature of M 2 InC (M = Zr, Hf and Ta) MAX phases. Low values of Vicker hardness indicate the softness of the materials and easy machinability.. The thermodynamic properties, such as the free energy, enthalpy, entropy, specific heat capacity and Debye temperature are evaluated using the phonon dispersion curves and a good correspondence is found with the M atomic species. Electronically important optical properties, e.g., dielectric functions, refractive index, photoconductivity, absorption coefficient, loss function and reflectivity are calculated and discussed in detail in this study. The term "M n+1 AX n phases" was coined by Barsoum in 2000 for the first time [1]. In the general formula M n+1 AX n with n = 1-3, M = early transition metal; A= which? group element and X= C and/or N). The MAX phases M 2 AX, M 3 AX 2 and M 4 AX 3 are referred as 211, 312, and 413 subject to the value of n. The M n+1 AX n phases are crystallized in the hexagonal structure belonging to the space group of P6 3 /mmc. The MAX phase nano layered ternary compounds are suitable for many technological applications owing to an unique combination of both metallic and ceramic properties. Like metals, they exhibit good electrical and thermal conductivity, machinability, low hardness, thermal shock resistance, and damage tolerance.On the other hand, these compounds possess ceramiclike high elastic moduli, high melting temperature, and oxidation and corrosion resistance [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. There are forty eight 211 phases that have already been listed without considering possible solid solutions, however more have been theoretically predicted [1,[18][19][20][21][22]. Furthermore, very recently the MAX phase materials are used as a precursor to synthesize atomically thin two-dimensional materials with many attractive physical features, the so called MXenes [23].The extensive research effort has been paid on both theoretical and experimental study of M 2 AX phases [2]...

First principles pseudopotential calculations have been performed for the first time to investigate the phonon dispersion, thermodynamic and optical properties including charge density, Fermi surface, Mulliken population analysis, theoretical Vickers hardness of predicted MAX phase Sc2InC. We revisited the structural, elastic, and electronic properties of the compound which assessed the reliability of our calculations. The analysis of the elastic constants and the phonon dispersion along with phonon density of states indicates the mechanical stability and dynamical stability of the Sc2InC. The Helmholtz free energy, internal energy, entropy specific heat capacity, and Debye temperature have also been calculated. Mulliken population analysis indicates the existence of prominent covalency in chemical bonding of Sc2InC. The electronic charge density mapping shows a combination of ionic, covalent and metallic bonding in the compound. The Fermi surface is comprised due to the low‐dispersive Sc 3d and C 2p states from the [ScC] blocks. The phase is expected to be a soft material and easily machinable due to its low Vickers hardness value. Furthermore, the analysis of various optical properties suggests that the nanolaminate Sc2InC is a promising candidate for optoelectronic devices in the visible and ultraviolet energy regions and as a coating material to avoid solar heating.