Local Structure Manipulates Nonlinear Optical Properties in Non-stoichiometric Lithium Niobate

Abstract

As a typical and commercial nonlinear optical (NLO) material, LiNbO3 usually suffers from non-stoichiometric compositions, but the role of non-stoichiometry in second harmonic generation (SHG) remains unclear. In this study, we prepared non-stoichiometric LiNbO3 with a Li mole percentage amount covering critical ranges from 47 to 50. We found that a 47 mol % Li amount can remarkably weaken the SHG response by 50% without an obvious change of the band gap. A detailed structural analysis indicates that reducing the Li content brings anti-occupied Nb (NbLi4·) together with Li vacancies (VLi′), which decreases local distortions and local polarizations of both NbO6 and LiO6 octahedra and ultimately lead to the decline of the SHG response. Such effects are discussed in terms of the looser and less anisotropic structure character of LiNbO3. The above results are supported by joint synchrotron X-ray/neutron diffraction, atomic pair distribution function (PDF) analysis, and theoretical calculations. These insights are helpful to the future design of functional materials in non-stoichiometric compositions.