Ytterbium charge state and stabilization in the Ba(Ca)F$_2$ host by electron paramagnetic resonance and infrared photoluminescence

TL;DR

This study systematically compares ytterbium charge state stabilization in BaF2 and CaF2 crystals, revealing how host lattice properties influence defect formation and optical behavior relevant for photonic and quantum technologies.

Contribution

It provides a detailed analysis of how host lattice differences affect Yb$^{2+}$ and Yb$^{3+}$ stabilization and defect mechanisms in fluoride crystals.

Findings

BaF2 favors Yb$^{3+}$ environments with dopant increase

CaF2 maintains predominantly unperturbed Yb sites

Host-specific optical responses observed in IR photoluminescence

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 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.