Insight into novel ErMI (M = S, Se) chalcohalide materials for next-generation energy applications

The study unveils an inclusive first-principles study of the electronic structure, optical, thermoelectric, and elastic properties of two novel rare-earth chalcohalides, ErSI and ErSeI, through density functional theory with the GGA + <i>U</i> approach, which includes spin-orbit coupling to account for strong 4f electron correlations. These systems are both dynamically and mechanically stable, having negative cohesive and formation energies, and fulfill all Born criteria for orthorhombic systems. An investigation of electronic band structure shows that ErSI and ErSeI are spin-polarized direct band gap semiconductors with notable exchange splitting and spin-channel asymmetry. ErSI has a larger band gap and enhanced localization of Er-4f states, whereas ErSeI has more dispersive bands, implying improved carrier mobility. Optical spectra suggest substantial absorption in the UV-visible region, with significant dielectric responses and plasmonic features; ErSI has greater dielectric constants and reflectivity, while ErSeI has higher refractive indices and larger interband transitions. Both materials exhibit negative Seebeck coefficients, indicating dominant n-type behavior. ErSI has a slightly greater figure of merit than ErSeI, due to its superior power factor. Mechanical study indicates that both compounds have equivalent ductility, with ErSeI exhibiting slightly larger elastic moduli, indicating greater mechanical resilience. These results suggest that ErSI and ErSeI are fascinating multifunctional materials with applications in thermoelectric modules, optoelectronics, and spintronic technologies.