We report on studies of optical absorption changes in congruent LiNbO3:Fe (0.2 at.%) crystals during heating in reducing (95%Ar+5%H2) and oxidizing (O2) atmospheres in the temperature (T) range of 273-1130 K. The kinetics recorded at fixed wavelength and T after rapid (~ 1 min) changes of gas atmospheres have been analyzed. The measurements in the spectral range of 50000–3333 cm–1 were performed using a special furnace located in the sample chamber of the Perkin-Elmer Lambda 900 spectrophotometer. The study was made on 0.8 mm-thick XZ-cut polished single crystalline plates.
The process of crystal coloration during heating in reducing atmosphere starts at about 600 К; at this temperature a band appears with its maximum near 20000 cm–1. The nature of this band is connected with the Fe3+-Fe2+ recharging processes. At about 800 K, the formation of another absorption band is observed with a maximum of 8000 cm–1, whose nature is connected with formation of free polarons. At sufficiently elevated temperatures, i.e. above 900 K, the shoulder of another absorption band is detected in the spectral range above 10000 cm-1 which is caused by the formation of bound polarons and bipolarons.
The growth of the absorption near 20000 cm–1 shows an activated behaviour. Analysis of the temperature dependence of the absorption at the maximum of this band allowed to evaluate the activation energy of the formation of Fe2+-ions, which is determined as 0.67±0.04 eV.
Replacement of the reducing atmosphere by an oxidizing one at fixed temperature leads to bleaching of the absorption bands, i.e. the crystal becomes colourless again. The coloration kinetics, recorded at different temperatures at 8000 cm-1 (maximum of the band, caused by the formation of free polarons) after a change of the atmosphere from O2 to 95%Ar+5%H2 are non-monotonous. This non-monotony can be adequately modelled by assuming, that the transformation of free polarons to the bound ones occurs during the reducing treatment.
The origin of the optical absorption changes caused by the reducing/oxidizing treatments is discussed in terms of the oxygen in- and out-diffusion to/from the material and the formation of the relevant point defects – oxygen vacancies, antisite niobium ions, and polarons. The obtained experimental and theoretical results are compared with those, published previously for undoped LiNbO3 and for LiNbO3:MgO (5 mol.%) [1, 2]