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

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Abstract
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 investigate the influence of the host cation, Ba2+ versus Ca2+, on the charge state, local environment, and incorporation behavior of ytterbium (Yb) dopants in BaF2 and CaF2 single crystals at low doping levels (0.05–0.2 mol%). 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 Yb3+ and Yb2+ 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 BaF2 favored perturbed Yb3+ environments with increasing dopant levels, while CaF2 maintained predominantly unperturbed sites, suggesting a more favorable ionic match for Yb2+. Photothermal deflection spectroscopy (PDS) and IR PL results showed host-specific optical responses, with CaF2 exhibiting crystal-field splitting and broader local field effects. These findings highlight the critical role of host cation identity in governing Yb charge state and lattice interaction, offering valuable insights for optimizing rare-earth-doped fluoride crystals in laser, scintillator, and quantum device applications.