Theoretical insights into the electronic, optical, and mechanical properties of 4c-Fe 7 Se 8 and 4c-(Fe 0.8 Cu 0.2 ) 7 Se 8 from first-principles calculations

Abstract This first-principles study investigates the electronic, optical, and mechanical properties of the 4c superstructure of Fe 7 Se 8 and its 20% Cu-doped variant, 4c-(Fe 0.8 Cu 0.2 ) 7 Se 8 , using density functional theory (DFT) with the PBE-GGA functional. The dielectric function ε ( ω ) = ε 1 ( ω ) + i ε 2 ( ω ) is analyzed over 0–30 eV, while detailed optical responses (refractive index, extinction coefficient, absorption coefficient, optical conductivity, loss function, and reflectivity) are examined in the 0–3.5 eV range. Undoped 4c-Fe 7 Se 8 exhibits a high refractive index and extinction coefficient, with a prominent absorption peak at 0.5–0.75 eV attributed to interband transitions involving dominant Fe 3d states near the Fermi level (as revealed by projected density of states). Cu doping reduces peak intensities, broadens spectral features, shifts responses, and enhances low-energy plasmonic activity (evident from the loss function). Reflectivity decreases from high metallic values toward near-transparency at higher energies, with further reduction upon Cu incorporation. These trends demonstrate tunable optical and plasmonic behavior via compositional modification. Mechanically, both compounds show strong elastic stiffness (Young’s modulus 176–178 GPa) and ductility (Poisson’s ratio ≈0.18), supporting potential use in stress-resilient structural applications. Compared to recent DFT studies on the 3c superstructure (where Cu doping enhances absorption intensity), the 4c phase displays distinct, superstructure-specific tunability due to differences in vacancy ordering and interlayer interactions. These insights provide a foundation for tailoring Fe 7 Se 8 -based materials for optoelectronic, plasmonic, and multifunctional applications.