Spatial distribution of optical coloration in single crystalline LiNbO3 after high-temperature H2/air treatments

The paper presents results of investigations of optical absorption spatial changes in congruent bulk LiNbO3 crystals in 300-800 nm spectral region along Х, Y, Z directions caused by high-temperature (400-700 оС) annealing in Н2 and, subsequently, in air. As it is shown, the coloration/discoloration processes are anisotropic: a maximal depth of coloration is observed for Z direction and a minimal one - for X direction. The reversibility of optical properties with respect to the annealing atmosphere changes as well as results of investigations by SIMS and IR spectroscopy methods suggest that the oxygen loss/incorporation. processes play a major role in the coloration/discoloration. The additional absorption spectrum caused by reduction consists of three bands, whose nature is associated with different point defects: 350-410 nm - defects in anion sublattice, 500-520 nm - bipolarons; 700-750 nm - bound polarons. Individual contributions of the bands to the additional absorption spectra depend on the annealing temperature and on the distance from the crystal surface. During reduction lithium oxide is leaving the crystal which leads to the formation of Li2O-depleted structural phase LiNb3O8 in near-surface regions. Experimentally obtained distribution of depth-dependent additional absorption in reduced crystals was successfully approximated basing on the oxygen vacancies diffusion model and solution of differential equations set describing a quasichemical reaction of bipolarons formation due to the oxygen loss in LiNbO3 crystal. On the other hand, this model does not allow to describe accurately the depth-dependent discoloration after oxidizing annealing of previously reduced lithium niobate crystals.