First-principles study of the electronic, optical, and thermodynamic behavior of TlYbSe₂ for advanced optoelectronic and thermodynamic applications

A first-principles study of the electronic, optical, and thermodynamic properties of thallium ytterbium selenide (TlYbSe₂) is carried out using density functional theory. Spin-polarized calculations within the PBE-GGA, TB-mBJ, and DFT+U frameworks are performed to examine the influence of strong Yb-4f electron correlations on the electronic structure. PBE-GGA and TB-mBJ predict half-metallic behavior of TlYbSe₂, where the spin-up channel shows semiconducting character and the spin-down channel shows metallic character. The DFT+U method properly captures the localization of Yb-4f electrons and reveals an indirect narrow band gap of approximately 0.62 eV, confirming semiconducting behavior. Volume optimization curves were computed using the Birch-Murnaghan equation of state to accurately determine the equilibrium structural parameters and bulk modulus. Phonon dispersion curves and phonon density of states were calculated to verify lattice dynamical stability. Optical properties show small anisotropy, with static dielectric constants ε₁(0) ≈ 10.0 (x-axis) and 12.5 (z-axis), absorption peaks at ≈ 3.5 eV (x-axis) and ≈ 5.0 eV (z-axis) in the visible and ultraviolet regions, refractive indices η(0) ≈ 3.20 (x-axis) and 3.5 (z-axis), and prominent plasmon resonances around 10 eV, while the absence of infrared intraband transitions further supports the semiconducting nature. Thermodynamic properties calculated using the quasi-harmonic Debye model show typical thermal expansion, lattice softening with temperature, Dulong-Petit type specific heat behavior approaching ≈ 100.0 J/mol·K, and a moderate Debye temperature θ D ≈ 181 K, indicating good thermal stability. Collectively, these results establish TlYbSe₂ as a strongly correlated narrow-band-gap semiconductor with promising potential for applications in optoelectronic, spintronic, and thermally tunable devices.