EPR results and Raman spectroscopy as a complementary characterization of isolated Yb ions and Yb pairs in CaF2:Yb single crystals

Raman spectrum of the CaF2:Yb crystal irradiated with γ-rays shows the localized phonon centred at about 64 cm−1 being an evidence of the ytterbium pairs presence. There are not observed absorption bands characteristic for Yb2+ ions in the absorption spectrum of “as-grown” (not irradiated or not annealed) CaF2:Yb (5 mol%) crystal.Electron paramagnetic resonance measurements of CaF2:Yb (5 mol%) single crystals were carried out for lattice sites substituted by Yb3+ ions and also for the possibility of ytterbium pairs arising. Among eight of EPR lines observed for “as-grown” crystal, mainly the lines originating from different Yb3+ isotopes can be distinguished: 170Yb3+I = 0, 171Yb3+I = 1/2 and 173Yb3+I = 5/2. The lines originating from Yb3+ pairs cannot be excluded. All of the lines do not move with angular variation in the planes parallel and perpendicular to the axis. The extra line in the magnetic field orientation in every plane at 45°, with B0 ∼ 191 mT is observed. This unidentified line corresponds to a non-axial Yb3+ centre or unknown impurity. The components of A and g tensors of the spin Hamiltonian for cubic symmetry were calculated.Temperature dependence of the EPR lines shows the agreement with the Curie law until the temperature of 18 K. The peak-to-peak linewidth changes continuously within 20 mT range for “as-grown” crystals, while shows significant linear increase above 18 K for γ-irradiated ones. It suggests the presence of Yb2+ ions, being created due to Compton electron capture. So, Yb3+ co-exists with Yb2+ after the Yb3+ → Yb2+ conversion under influence of γ-irradiation. It seems the Yb2+ centres arising in CaF2:Yb (5 mol%) single crystal irradiated with γ-quanta are related to Yb3+, as an effect of recharging one of Yb3+ ion from a pair, or form Yb3+–Yb2+ pair when Compton electron is captured by one of isolated neighbour Yb3+ ions.