Energy band gap and optical properties of lithium niobate from ab initio calculations

The electronic and optical properties of stoichiometric lithium niobate are calculated from first principles within the Generalized Gradient Approximation (GGA). The band gap is found slightly indirect. Its value, underestimated by the GGA, is corrected on the basis of the refractive index by using our new approach called reverse scissor correction procedure. The corrected value (4.9 eV) is fairly comparable to previous ones obtained by GW calculation. The frequency dispersions of the dielectric function are re-calculated for different scissor corrections and compared to the experimental Sellmeier equation. A good agreement is obtained in the NIR-VIS-NUV range with our corrected band gap. The hopping integrals and effective masses at the extrema of the conduction and valence bands are also estimated.