Ce3+ to Ce4+ Recharge in Ce3+ Doped and Ca2+–Si4+ and Mg2+–Si4+ Codoped Y3Al5O12 Single Crystalline Film Phosphors: Optical and EPR Studies

Abstract

Single-crystalline film (SCF) phosphors of Ce-doped Y3Al5O12 (YAG:Ce) codoped with heterovalent Ca2+–Si4+ and Mg2+–Si4+ ion pairs were grown by liquid phase epitaxy from PbO–B2O3-based flux and systematically investigated using optical spectroscopy, scintillation measurements, and electron paramagnetic resonance (EPR). The influence of heterovalent codoping on the Ce3+/Ce4+ charge state balance, local structure of Ce-related centers, and scintillation performance was analyzed. Absorption and EPR spectra reveal that excess Ca2+ or Mg2+, and Pb2+ ions promote efficient Ce3+ → Ce4+ recharging, while partial compensation by Si4+ ions induces limited reverse Ce4+ → Ce3+ conversion. As-grown Ca–Si and Mg–Si codoped SCFs are therefore dominated by Ce4+ states, leading to reduced scintillation light yield but significantly accelerated photoluminescence and scintillation decay kinetics compared to YAG:Ce SCF. Thermal treatment in a reducing atmosphere (N2/H2) partially restores Ce3+ via oxygen-vacancy formation, resulting in increased light yield (up to ∼65–70% of YAG:Ce) and more exponential decay behavior. The combined optical and EPR results confirm the formation of multiple Ce3+ centers with distinct local environments caused by compositional disorder. The important role of Ce4+ centers in charge compensation and fast excitation pathways is demonstrated, providing insight into charge transfer, trapping, and scintillation mechanisms in SCFs of mixed garnet. These findings are relevant for the design of advanced scintillators and high-power LED converters based on compositionally engineered garnet structures.