Substitutional impact of alkali metals on the electronic structure, optical, mechanical, and transport properties of novel ternary materials

This study investigates the structure, optoelectronic, mechanical, and transport features of two novel NaAcTe₂ and RbAcTe₂ chalcogenides, using first-principles calculations. Both materials crystallize in the cubic F $\bar{m}$ 3 m phase, while RbAcTe₂ has higher thermodynamic stability because of its negative formation energy (−3.52 eV/atom) and larger lattice constants. TB-mBJ electronic band structure modelling indicates that both compounds are direct band gap semiconductors, with RbAcTe₂ (1.62 eV) having a larger energy gap than NaAcTe₂ (1.29 eV). The density of states analysis indicates that Te-p and Ac-f/d contributions are prevalent near the Fermi level, suggesting favorable electronic transitions and improved thermoelectric response. NaAcTe₂ has stronger dielectric function peaks, greater absorption in the 4–12 eV range, and larger reflectivity, indicating better light-matter interaction and plasmonic performance. Mechanical investigation shows that both materials are ductile and mechanically stable. RbAcTe₂ has greater elastic constants although NaAcTe₂ is more ductile. NaAcTe₂ has greater electrical conductivity and ZT = 0.32 at 700 K) compared to RbAcTe₂, which has a higher Seebeck coefficient. These results demonstrate that NaAcTe₂ is appropriate for optical and thermoelectric devices that require a balance of mechanical flexibility and conductivity, whereas RbAcTe₂ is well-suited for high-stability, rigid device configurations.