Abstract
The influence of composition on the photorefractive effect in pure LiNbO3 crystals at low light intensity was investigated. The experimental results indicate that different defects dominate the photorefractive centers of pure LiNbO3 with various compositions. Bipolarons are considered to be responsible for the enhanced photovoltaic field in reduced near-stoichiometric LiNbO3, and their bulk photovoltaic constant kappa is estimated to be approximately 6.95 x 10(-32) m3/V. Q polarons (composed of two bipolarons) are introduced to explain the photorefractive effect of congruent LiNbO3 at both low and high light intensities.