Defect complexes in congruentLiNbO3and their optical signatures

Abstract

The structure and stability of defect clusters in ${\mathrm{LiNbO}}_{3}$, as well as their influence on the linear and nonlinear optical susceptibilities, are calculated within density functional theory (DFT) using semilocal and hybrid exchange-correlation functionals. In particular, the complexes modeling the Li shortage during the crystal growth, the Li-vacancy model and the Nb-vacancy model, are examined in detail. It is found that clustering significantly decreases the formation energies of all considered defects with respect to the dilute limit. The Li-vacancy model is energetically preferred with respect to the total formation energy, while the Nb-vacancy model has the lowest formation energy per single point defect. The independent-particle approximation based on the hybrid DFT electronic structure describes the ${\mathrm{LiNbO}}_{3}$ optical response much better than semilocal DFT. A further improvement between the calculated optical absorption and second-harmonic generation spectra with experiment is achieved if the calculations take defect complexes into account. Nb antisite polarons give rise to optical absorption within the band gap.