Electron paramagnetic resonance spectra of La<sub>3</sub>Ga<sub>5.5</sub>Ta<sub>0.5</sub>O<sub>14</sub>:Ho single crystal for the presence of Yb<sup>3+</sup> magnetic nonequivalent centers are analyzed. Centers of different symmetries we found two of which being cubic symmetry sites. The existence of the cubic symmetry centers of the Yb<sup>3+</sup> ions in LGT host crystal confirm that Yb<sup>3+</sup> ions occupies eightfold coordinated La<sup>3+</sup> ions. The values of the g-factor for these centers are estimated as: <i>g</i> = 2.059 ± 0.004 (I centre) and <i>g</i> = 2.840 ± 0.015 (II centre). Some results of optical measurements are also presented. They indicate sharp but a weak Yb<sup>3+</sup> absorption centered at about 978 nm in the IR part of the absorption spectrum, and, equally weak Ho<sup>3+</sup> absorption in the UV-VIS part of the spectrum (fundamental absorption edge being equal to 250 nm). We think Ho and Yb ions compete in substituting of lattice sites in the crystal. Photoluminescence measurements have shown a very strong signal in the green (533 - 555 nm), slightly less in red (637 - 671 nm) and near-IR (735 - 769 nm) spectral ranges. Co-activation of the Yb-doped crystals with holmium ions leads to appearance of visible luminescence, which is explained by the Yb<sup>3+</sup>-Ho<sup>3+</sup> stepwise up-conversion mechanism. Additional absorption measurements performed after γ-irradiation with a dose of 10<sup>5</sup> Gy show wide band originating at 250 nm and extending up to 500 nm (probably due to recharged cation and oxygen vacancies) for both LGT pure and LGT:Yb, Ho single crystals, the lowering of the amount of Yb<sup>3+</sup> and Ho<sup>3+</sup> ions (due to Compton electrons capture) and additional absorption band centered at about 600 nm (charge compensating defects) for LGT:Yb, Ho single crystals. Thermoluminescence measurements (TL), does not reveal the presence of any traps in the TL glow curve.
Electron paramagnetic resonance (EPR) measurements of LiNbO<sub>3</sub>: Yb, Pr (0.8wt.%, 0.1 wt.%) single crystals were analyzed for lattice sites of Yb in the crystal and also for the Yb<sup>3+</sup> pairs arising. Parameters of the spin Hamiltonian were calculated using EPRNMR program. From the angular variations of the EPR spectra it results Yb<sup>3+</sup> ions of C<sub>1</sub> symmetry arise in the crystal. Pr<sup>3+</sup> ions change parameters of spin hamiltonian for Yb<sup>3+</sup> ion. Some results of the absorption measurements were also analyzed for UV-VIS and IR giving evidence, together with EPR results, on Yb<sup>3+</sup> location at Li<sup>+</sup> sites and Pr<sup>3+</sup> location at Nb<sup>5+</sup> sites. For comparison absorption spectra of the LiNbO<sub>3</sub> doped with 0.5 wt. % Pr<sup>3+</sup>, 0.8 wt. % Yb<sup>3+</sup> were analyzed. Low temperature absorption measurements have shown the presence of low energy phonons responsible for the intensity of the main Yb<sup>3+</sup> absorption line in IR as a function of the temperature. Raman spectra confirmed the observation.