Exploring multifunctional properties of ternary chalcogenides for advanced energy applications

We performed a thorough first-principles study of Mg₂TeS and Mg₂TeSe, including structural, electronic, optical, and transport features. Structurally, both compounds have a trigonal R3m structure with different polyhedral designs. Mg₂TeSe shows a larger equilibrium volume, and thermodynamic parameter (Ecoh ≈ - 3.65 eV/atom for Mg₂TeS and - 3.79 eV/atom for Mg₂TeSe; ΔHf ≈ - 2.17 and - 2.53 eV/atom) indicate both are energetically valuable, with Mg₂TeSe relatively more stable. Electronically, these materials are found as direct band gap semiconductors. The WC-GGA underestimated band gaps with values of (≈ 1.49 eV and 1.83 eV), which were then corrected by SOC + TB-mBJ with values of (≈ 2.64 eV and 2.71 eV) for Mg₂TeS and Mg₂TeSe, respectively. Projected density of states indicates valence bands are dominated by Te states and conduction bands by Mg states, and replacing S with Se narrows the band gap while shifting density of states toward the Fermi level. Optically, notable interband responses occur with ε₁(ω) and refractive index maxima between 5.0 and 5.5 eV, ε₂(ω) peaks ≈ 5.5-5.8 eV, static refractive index ≈ 2.1, absorption peaking at ≈ 4.0 eV (Mg₂TeS) and ≈ 3.5 eV (Mg₂TeSe), low static reflectivity (~ 0.12), and plasmonic loss peaks near 17.0 and 16.0 eV. The transport results demonstrate modest Seebeck at low T, electrical conductivity σ/τ ≈ 2.27 × 10¹⁹ and 2.24 × 10¹⁹ (Ω·m·s)⁻¹ at 300 K, lower lattice thermal conductivity and a greater power factor for Mg₂TeSe, and ZT increasing with temperature to maxima ≈ 0.32 (Mg₂TeS) and 0.24 (Mg₂TeSe) at 1200 K, illustrating Mg₂TeSe's promise for thermoelectric applications.