Coupled charge-spin-photon dynamics in Ce/Tb Co-doped CaLa4Si3O13: Toward quantum-level design of multifunctional phosphors

Efficient white light-emitting diodes (w-LEDs) require phosphors that combine spectral tunability, thermal stability, and structural robustness. Among silicate hosts, CaLa4Si3O13 (CLSO) provides flexible cationic sites for rare-earth activation, making it an ideal platform for exploring Ce3+/Tb3+ co-doping. Although experiments have demonstrated efficient Ce -> Tb energy transfer and high luminescence yield, the atomic-scale mechanism driving this synergy has remained unresolved. In this work, we employ all-electron density-functional theory (WIEN2k, FP-APW + lo) with GGA + U + SOC and the TB-mBJ potential to analyze the electronic, structural, and optical effects of Ce3+/Tb3+ substitution in CLSO. The calculations show a self-compensating charge transfer from Ce3+ (donor) to Tb3+ (acceptor), decreasing the defect-formation energy by about 0.2 eV per formula unit and improving lattice stability through balanced electrostatics. This donor-acceptor pairing reduces the band gap, creates spin-polarized mid-gap states, and boosts visible-range absorption, aligning with observed photoluminescence shifts. Electron localization and elastic analyses confirm enhanced bonding flexibility and moderate phonon softening without causing dynamic instability. The results reveal a microscopic mechanism that connects charge compensation to optical and magnetic functionalities, providing a design strategy for co-doped oxides where complementary dopants ensure both thermodynamic stability and optical efficiency.