EPR and optical properties of La3Ga5.5Ta0.5O14:Yb3+ and Ho3+ single crystals

Electron paramagnetic resonance spectra of La3Ga5.5Ta0.5O14:Ho single crystal for the presence of Yb3+ 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 Yb3+ ions in LGT host crystal confirm that Yb3+ ions occupies eightfold coordinated La3+ ions. The values of the g-factor for these centers are estimated as: g = 2.059 +/- 0.004 (I centre) and g = 2.840 +/- 0.015 (II centre). Some results of optical measurements are also presented. They indicate sharp but a weak Yb3+ absorption centered at about 978 nm in the IR part of the absorption spectrum, and, equally weak Ho3+ 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 Yb3+-Ho3+ stepwise up-conversion mechanism. Additional absorption measurements performed after gamma-irradiation with a dose of 10(5) 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 Yb3+ and Ho3+ 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.