Optimized energy transfer and enhanced green photoluminescence in Tb³⁺/Gd³⁺ co-doped BiF₃ nanoparticles via...

Tb³⁺/Gd³⁺-codoped BiF₃ nanoparticles were synthesized via a controlled, room-temperature co-precipitation method to address the critical challenge of low photoluminescence (PL) quantum efficiency in singly Tb³⁺-doped fluoride phosphors. Structural analysis confirmed a stable cubic phase (Fm3̄m) with successful substitutional doping on Bi³⁺ sites. At the optimal 5 : 5 mol% Tb : Gd ratio, a quantum yield of 13.5%-a fourfold enhancement over Tb³⁺-only systems-was achieved. The mechanism of enhancement operates through two complementary, excitation-wavelength-dependent pathways: (I) Under short-UV irradiation (230-260 nm), Gd³⁺ acts as a sensitizer-harvesting photons via its ⁸S_7/2 → ⁶I_J/⁶P_J transitions and transferring energy to Tb³⁺via resonance Gd³⁺ → Tb³⁺ energy transfer, directly proven by excitation spectroscopy. (II) Under 365 nm excitation-where Tb³⁺ is the primary absorber and Gd³⁺ absorbs negligibly-Gd³⁺ enhances PL efficiency through a complementary crystal field modification mechanism: Gd³⁺ substitution modifies the local phonon environment around bulk Tb³⁺ sites, reducing the non-radiative depopulation rate of the ⁵D₄ excited state. This is directly evidenced by time-resolved PL decay showing +65% and +87% elongation of Tb³⁺ bulk lifetime components (τ₂ and τ₃) upon co-doping. CIE chromaticity analysis confirms high-purity green emission. These findings provide dual-mechanism characterization of Gd³⁺/Tb³⁺ co-doping in BiF₃, establishing a rational design framework for Gd³⁺-enhanced lanthanide phosphors.