Enhanced Thermo-Mechanical Stability of Sc ₂ TlC and Sc ₂ PbC MAX-Phase Carbides under Hydrostatic Pressure

This work presents a first-principles investigation of the pressure-dependent structural, elastic, thermal, and optical properties of the layered MAX-phase carbides Sc2XC (X = Tl, Pb) using density functional theory under hydrostatic pressures ranging from 0 to 12 GPa. The evolution of elastic wave velocities, Debye temperature, melting temperature, and lattice thermal conductivity is systematically analyzed to assess the thermo-mechanical stability of these compounds under compression. At ambient pressure, Sc2TlC exhibits longitudinal, transverse, and average sound velocities of 4725.66 m s -1 , 2653.43 m s -1 , and 2952.76 m s -1 , respectively, which increase markedly to 5619.72 m s -1 , 3219.24 m s -1 , and 3576.39 m s -1 at 12 GPa, accompanied by an enhancement of the Debye temperature from 317.99 K to 400.93 K. Similarly, Sc2PbC shows a substantial rise in Debye temperature from 275.52 K to 372.13 K over the same pressure range, indicating improved lattice stiffness and phonon stability. The lattice thermal conductivity at 300 K increases significantly with pressure, reaching 73.56 W m -1 K -1 for Sc2TlC and 60.24 W m -1 K -1 for Sc2PbC at 12 GPa, reflecting enhanced phonon transport under compression. Optical properties reveal pronounced anisotropy, characterized by exceptionally high extinction coefficients along the xx direction (K xx ≈ 58-60 at eV) and negative refractive indices along the zz direction (n zz ≈ -7.5 at 0 eV), indicative of metallic and plasmonic behavior. Overall, the results demonstrate that Sc 2 TlC and Sc 2 PbC combine excellent mechanical robustness with pressure-tunable thermal transport and strongly anisotropic optical responses, highlighting their potential for high-temperature structural applications, advanced thermal management, plasmonic devices, and anisotropic optoelectronic technologies.