We present a band structure approach with a molecular dynamics cluster optimization which accounts for the various structural modifications related to the non-stoichiometry of LiNbO₃ crystals. The variation of the optical properties with the deviation from the stoichiometric composition can be understood within this approach. Particular role of the electron-phonon contributions to the electrooptics coefficient is shown. Model calculations yield a large dependence of the electrooptis coefficient r₂₂ on the crystal composition, in agreement with the experimental data. The observed minimum of the r₂₂ coefficient versus the non-stoichiometry is interpreted as originated from the non-centrosymmetry in the electrostatic potential distribution around Nb-O₆ clusters.

Keywords: niobate of lithium; defect in crystals

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