Luminescence tuning via Eu2+/Eu3+ balance in CaF2 particles synthesized via microwave-assisted hydrothermal method

ABSTRACT: A rapid microwave-assisted hydrothermal (MAH) route combined with short post-thermal treatments is demonstrated as an effective strategy to control defect structure and europium valence in CaF 2 :Eu particles. Eu-doped CaF 2 particles were synthesized using a 1 min MAH process and subsequently subjected to thermal treatment at different temperatures. This work investigates how annealing influences the defect structure and, consequently, the balance between Eu 3+ and Eu 2+ emission in a cubic CaF 2 host. X-ray diffraction combined with Rietveld refinement confirms the formation of a single-phase cubic structure without secondary phases, while scanning electron microscopy reveals a particle growth from 65 to 137 nm, without significant morphological changes. Fourier transform infrared (FTIR) analysis indicates the progressive removal of hydroxyl and nitrate species upon annealing, reflecting modifications in the local chemical environment and suggesting the evolution of defect-related states in the CaF 2 lattice. EPR provides evidence of paramagnetic centers, while XPS confirms the coexistence of Eu 2+ and Eu 3+ ions and shows that thermal treatment modifies their relative proportions, with a maximum Eu 2+ fraction of 58.10% for samples treated at 600 °C. These changes strongly affected the optical response: photoluminescence measurements showed distinct excitation-dependent emission pathways, whereas radioluminescence under X-ray excitation revealed enhanced blue emission associated with Eu 2+ together with contributions from intrinsic defect centers. Overall, the results demonstrate that thermal treatment promotes defect-mediated control of the Eu 2+ /Eu 3+ balance, enabling tuning of the luminescent response of CaF 2 :Eu materials. This approach provides a simple and effective strategy for optimizing fluoride-based systems for applications involving high-energy excitation, such as scintillation and radiation detection.