Abstract
Raman spectrum of the CaF 2 :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 Yb 2+ ions in the absorption spectrum of as-grown (not irradiated or not annealed) CaF 2 :Yb (5 mol%) crystal. Electron paramagnetic resonance measurements of CaF 2 :Yb (5 mol%) single crystals were carried out for lattice sites substituted by Yb 3+ 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 Yb 3+ isotopes can be distinguished: 170 Yb 3+ Ι = 0, 171 Yb 3+ 1= 1/2 and 1?3 Yb 3+ Ι=5/2. The lines originating from Yb 3+ 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 B 0 ∼ 191 mT is observed. This unidentified line corresponds to a non-axial Yb 3+ 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 Yb 2+ ions, being created due to Compton electron capture. So, Yb 3+ co-exists with Yb 2+ after the Yb 3+ → Yb 2+ conversion under influence of γ-irradiation. It seems the Yb 2+ centres arising in CaF 2 :Yb (5 mol%) single crystal irradiated with γ-quanta are related to Yb 3+ , as an effect of recharging one of Yb 3+ ion from a pair, or form Yb 3+ -Yb 2+ pair when Compton electron is captured by one of isolated neighbour Yb 3+ ions.
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