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Electron paramagnetic resonance (EPR) measurements of CaF2:Yb (5 mol. %) single crystals were performed for lattice sites substituted by Yb ions and also for the possibility of Yb3+ pairs arising. Among eight lines observed in the EPR spectrum, with g = 4.293, 4.1, 3.965, 3.665, 3.433, 3.13, 3.054, 2.894 we think mainly the lines coming from different isotopes of Yb3+ isolated ions of cubic symmetry one can distinguish: 170Yb3+ I=0, 171Yb3+ I=1/2 and 173Yb3+ I=5/2, but one can not exclude the lines coming from Yb3+ pairs. All of the lines have got an axial symmetry along the crystallographic axis, since the lines do not move with angular variation in the planes parallel and perpendicular to the axis. One can observe also the extra line in magnetic field orientation in every plane at 45°, with the B0≈191 mT. The unidentified line corresponds to non-axial Yb3+ center or unknown impurity. The components of A and g tensors of the spin Hamiltonian for cubic symmetry were calculated. Temperature dependence of EPR lines shows agreement with the Curie law for most of the lines. Peak-to-peak linewidth changes continuously within 20 mT range for "as-grown" crystals, while shows distinct increase above 25 K for γ-irradiated ones. It suggests strong ferromagnetic coupling between neighbours Yb3+ and Yb2+ ions the latter being created due to Compton electron capture. So, Yb3+ co-exists with Yb2+ after the Yb3+-Yb2+ conversion under influence of γ-irradiation and/or annealing in hydrogen. It seems the Yb2+ centers arising in CaF2:Yb (5 mol. %) single crystal after γ-irradiation are related to Yb3+, as an effect of recharging one of Yb3+ ion from a pair. Absorption measurements of CaF2 highly doped with Yb3+ (0.5 mol. %, 5 mol. % and 30 mol. %) reveal the shifting of the IR maxima of 921 and 980 nm characteristic for 0.5 mol. % doping of Yb3+ towards the centrum of IR band (927 and 976 for 30 mol. % of Yb3+). Raman spectrum shows localized phonon centered at about 64 cm-1 being confirmation of the Yb3+ pairs presence in the 5 mol. % of Yb3+ doped CaF2 crystal.
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