Influence of non-stoichiometric defects on optical properties in LiNbO3

We present a band structure approach with a molecular dynamics cluster opti mization which accounts for the various structural modifications related to the non-stoichiometry of LiNbO3 crystals. The variation of the optical pro perties with the deviation from the stoichiometric composition can be under stood within this approach. Particular role of the electron-phonon contribu tions to the electrooptics coefficient is shown. Model calculations yield a large dependence of the electrooptis coefficient r(22) on the crystal comp osition, in agreement with the experimental data. The observed minimum of t he r(22) coefficient versus the non-stoichiometry is interpreted as origina ted from the noncentrosymmetry in the electrostatic potential distribution around Nb-O-6 clusters.