We report large and nonmonotonic dependences of the electro-optic (EO) coefficient ${{r}}_{22}$ and the corresponding dielectric permittivity in pure ${\mathrm{LiNbO}}_3$ on the concentration of intrinsic defects related to the deviation from the stoichiometry of the crystal. This variation is caused mainly by the electro-mechanical contribution. A stoichiometric crystal possesses both large EO properties and resistance to optical damage which renders it much more suitable for use in laser Q-switching than the congruent crystal.

We present a band structure approach in connection with a molecular dynamics cluster optimization that accounts for various structural modifications related to the nonstoichiometry of ${\mathrm{LiNbO}}_3$ crystals. Variation of optical properties with the deviation from the stoichiometric composition can be understood within this approach. The particular role of the electron-phonon contributions to the electro-optic coefficient is shown. Model calculations yield a large dependence of the electro-optic coefficient $r_{22}$ on the crystal composition, in agreement with the experimental data. The observed minimum of the $r_{22}$ coefficient versus nonstochiometry is explained within a microscopic approach. The $r_{33}$ tensor component shows substantially different behavior versus the stoichiometry.

In spite of the large utilization of LiNbO₃ in optical devices, its electro‐optic coefficients are not perfectly known. We show in the present paper that not taking into account the temperature dependence of the linear spontaneous birefringence can lead to an erroneous determination of the electro‐optic coefficients. The electro‐optic coefficient r_c of LiNbO₃ has been determined by an accurate technique issued from the Sénarmont method. At room temperature, r_c=19.9×10⁻¹² m/V, with an estimated error of ±5%. The measurements are performed for values of the dc applied electric field as a function of the temperature around room temperature. We point out the drastic influence on the electro‐optic response of the spontaneous birefringence variation due to the temperature. Accurate and reproducible values of the electro‐optic coefficients are obtained only if this dependence is taken into account.