Lanthanide-Induced Local Structural and Optical Modulation in Low-Temperature Ag2Se

Low-temperature Ag2Se is a narrow-band semiconductor, with its transport and optical properties significantly influenced by the local coordination environment. This study investigates the effects of La and Gd incorporation using DFT+U calculations and Ag-K edge EXAFS analysis. Analysis of electron localization function (ELF) and charge density differences reveals increased electron localization at dopant sites. Additionally, k3χ(k) and wavelet transforms demonstrate that the first M-Se shell shifts from approximately 1.346 Å in Ag-Se to around 1.386 Å and 1.291 Å for La-Se and Gd-Se, respectively (phase-uncorrected), thereby confirming dopant-specific lattice distortions while maintaining the orthorhombic framework. The observed changes are associated with an increase in dielectric strength, with ε2 increasing from approximately 30–40 in pristine Ag2Se to around 50–60 for La and 70–80 for Gd at low photon energies, alongside enhanced absorption nearing 1.32–1.34 × 105 cm−1. The characteristic plasmon resonance in the range of 15–20 eV is maintained. Rare-earth substitution effectively adjusts local bonding and low-energy optical response in Ag2Se, with Gd demonstrating the most significant impact among the examined dopants.