In a survey of recent publications, the relationship between photore-fraction and defect structure is outlined. We start with a description of the situation in undoped lithium niobate at moderate temperature. Changes in spectral characteristics (optical absorption, EPR) caused by reduction treatments and their interpretation using the polaron model are discussed then. The characteristics are used as tests for Nb antisites, i.e., Nb on an Li site. Hydrogen is the first candidate to form an extrinsic defect influencing via the protonic conductivity the charge transport (photorefraction, thermal fixation). In a next step the photorefractive impurities like the transition metals Fe, Cu, Mn, and Ni are discussed in detail, with special emphasis on iron. Some interesting features of Cr and lanthanide ions supplement this topic. Optical-damage-resistant impurities (Mg, Zn, Sc, In, Hf, and Zr) are antagonists of Nb antisites and strongly reduce the photorefraction, when doping above certain threshold concentrations. These dopants increase the phase transition temperature, the density and shift the bandedge absorption towards the UV. Many properties differ below and above the threshold concentrations, which strongly depend on the crystal composition. Near stoichiometric samples have thresholds at much lower concentrations then congruent ones. A table with energies of the OH stretch mode in lithium niobate closes this chapter.
Point Defects in LiNbO3. (2009). In Springer Series in Materials Science (Vol. 115, pp. 9–50). Springer Verlag. https://doi.org/10.1007/978-3-540-70766-0_2