Structural, thermomechanical, and transport properties of Ti ₂ NbSiC ₂ and Ti ₂ MoSiC ₂ MAX phases as candidates for high‐temperature structural and coating materials

Abstract We report a first‐principle predictive investigation of the quaternary MAX phases Ti 2 NbSiC 2 and Ti 2 MoSiC 2 spanning structure, elasticity, electronic and thermodynamic behavior, and electronic transport response. To the best of our knowledge, these compositions have not yet been synthesized experimentally, so this work assesses their stability and properties on a purely computational basis. Variable‐cell relaxations identify the stacking as the ground state for both chemistries, with reduced equilibrium volume and larger Bulk modulus than the polymorph. Formation energies, satisfaction of the hexagonal Born stability criteria, and the absence of imaginary modes in the phonon dispersions of the polytypes jointly indicate thermodynamic and dynamical stability at K. The single‐crystal elastic tensors satisfy the Born stability criteria and, after Voigt–Reuss–Hill reduction, reveal a clear partitioning: Ti 2 NbSiC 2 exhibits the larger shear modulus and Vickers hardness, whereas Ti 2 MoSiC 2 shows a higher bulk modulus and falls on the ductile side of the empirical Pugh/Poisson criteria. Directional Cauchy pressures and anisotropy factors (–, ) indicate stronger covalent character and prismatic‐over‐basal shear in both phases. Band structures and densities of states confirm metallicity with dominant transition‐metal states hybridized with C and Si near ; Ti 2 MoSiC 2 presents the larger . From elastic‐wave velocities, we obtain Debye temperatures (Ti 2 NbSiC 2 ) and K (Ti 2 MoSiC 2 ) and room‐temperature lattice thermal conductivities of 27 and 20 W , respectively. These thermal‐transport values are obtained within the quasi‐harmonic Debye framework and a Slack‐type model and thus should be interpreted primarily in terms of comparative trends. Boltzmann transport (constant‐relaxation‐time) yields negative Seebeck coefficients over 300–1500 K with larger and for Ti 2 MoSiC 2 ; the figure of merit remains below 0.02 owing to substantial heat conduction and is therefore much smaller than in state‐of‐the‐art thermoelectric materials. Taken together, these results indicate that Ti 2 NbSiC 2 and Ti 2 MoSiC 2 should be regarded primarily as high‐temperature structural and electrically/thermally conductive coating materials, with their thermoelectric coefficients serving mainly as probes of the underlying electronic structure. Accordingly, we do not pursue thermoelectric optimization strategies here. The combined data position Ti 2 NbSiC 2 for stiffness‐critical conductive components and coatings at elevated temperature, with Ti 2 MoSiC 2 favored where greater mechanical compliance is beneficial. Future work that explicitly treats finite‐temperature elasticity and anharmonic phonon scattering (phonon lifetimes) would further refine the predicted high‐temperature mechanical and thermal‐transport behavior.