Multifrequency EPR/ENDOR/optical study of ytterbium centers in stoichiometric lithium niobate

The tremendous narrowing of lines of Electron Paramagnetic Resonance (EPR) in nearly stoichiometric lithium niobate samples, when compared to those in congruent samples, allowed us to distinguish nine non-equivalent centers, as well as line splitting caused by the hyperfine interaction of ytterbium electrons with the nuclear spins of $^{171}$Yb and $^{173}$Yb. Three Yb$^{3+}$ centers have axial C$_{3}$ symmetry; all others have the lowest C$_{1}$ symmetry due to the presence of intrinsic defects and/or charge compensation defects in the near neighborhood of Yb$^{3+}$. Our study of Electron Nuclear Double Resonance (ENDOR) gave direct evidence that Yb$^{3+}$ in the main axial center substitutes for Li$^{+}$ and has no other defects in its surrounding (distant charge compensation mechanism). Possible models for low-symmetry centers are proposed. The obtained numerous characteristics of \textbf{g}-tensors and hyperfine tensors can be used as cornerstones for model calculations of Yb$^{3+}$ centers in lithium niobate.