Ytterbium charge state and stabilization in the Ba(Ca)F2 host by electron paramagnetic resonance and infrared photoluminescence

Lanthanide-doped fluorides are promising materials for advanced photonic and quantum applications due to their wide bandgap, low phonon energy, and chemical stability. In this work, we present a systematic comparative study of ytterbium incorporation at low doping levels (0.05-0.2 mol%) in BaF 2 and CaF 2 single crystals, focusing on the interplay between host lattice properties, charge-state stabilization, and defect formation mechanisms. Using a combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), transmittance, and infrared photoluminescence (IR PL), we explore how host lattice properties affect the stabilization of Yb 3+ and Yb 2+ ions. XRD confirmed cubic phase purity and lattice parameter stability in both hosts, while XPS revealed surface chemical composition variations associated with charge-compensating defects and trace impurities. EPR spectra indicated that BaF 2 favored perturbed Yb 3+ environments with increasing dopant levels, while CaF 2 maintained predominantly unperturbed sites, suggesting a more favorable ionic match for Yb 2+ . Photothermal deflection spectroscopy (PDS) and IR PL results showed host-specific optical responses, with CaF 2 exhibiting crystal-field splitting and broader local field effects. These results reveal a clear decoupling between long-range structural stability and local lattice perturbations, and demonstrate that host cation identity governs the balance between Yb 2+ and Yb 3+ stabilization as well as defect-driven optical behavior. This offers valuable insights for optimizing rare-earth-doped fluoride crystals in laser, scintillator, and quantum device applications.