First-principles investigation of the structural, optoelectronic, and thermoelectric properties of Sr2MgXO6 (X = S, Se) double perovskite materials

The physical properties of the chalcogenide-based double perovskites Sr₂MgXO₆ (X = S, Se) were investigated in the cubic phase using first-principles density functional theory. Structural stability was assessed through formation energy analysis and ab initio molecular dynamics simulations, confirming both dynamical and thermal stability of the compounds. The electronic structure was calculated using the full-potential linearized augmented plane wave method in combination with the Tran–Blaha modified Becke–Johnson potential, yielding improved band-gap values of 1.4 eV for Sr₂MgSO₆ and 2.2 eV for Sr₂MgSeO₆. Real-space bonding characteristics were examined using electron localization function, charge density difference, and Bader charge analyses, revealing dominant ionic interactions with moderate covalent contributions within the chalcogen–oxygen framework. Carrier transport properties were evaluated through electron and hole effective mass calculations, indicating relatively favorable electron mobility in both materials. Using the effective masses and static dielectric constants, exciton binding energies were estimated and found to be sufficiently low to enable efficient exciton dissociation at room temperature. Optical properties, including dielectric response, absorption coefficient, reflectivity, energy loss function, and refractive index, demonstrate strong absorption across the visible to ultraviolet regions with low optical losses. The combined electronic, bonding, excitonic, and optical characteristics highlight Sr₂MgXO₆ (X = S, Se) as promising candidates for photovoltaic and optoelectronic applications. Thermolectric study suggests that the materials are p-type semiconducting due to their positive Seebeck values. The PF and ZT analyses indicate that Sr₂MgSeO₆ exhibits higher thermoelectric performance compared to Sr₂MgSO₆ material.