Abstract
Pure lithium niobate crystals with various Li compositions are prepared by the vapour-transport-equilibration technique. UV-light-induced absorptions (ULIAs) in these crystals are detected by the probe light with a wavelength of 780 nm, and the composition dependence of the ULIA is studied. It is found that the ULIA increases slowly with the composition below 49.70 mol% but rises up very rapidly above this critical composition. These findings are explained by the two different stages in the evolution of photorefractive centres during the increase in the crystal composition. In addition, the thermal effect of ULIA is observed and attributed to the transfer of electrons from small polarons to the conduction band by thermal excitation.