Lone pair electron effect induced differences in linear and nonlinear optical properties of bismuth borates

Abstract

The Bi(III) cations associated with lone pair electrons can help to enhance the linear (birefringence) and nonlinear optical (second harmonic generation, SHG) performances. In this work, the influence of bismuth cations on birefringence or SHG in α, β, γ, δ, and ε-BiB₃O₆, BiB₂O₄F and BiB₂O₄(OH) is investigated via first-principles, real-space atom-cutting and lone pair electron analysis methods. It is revealed that the intensity of lone pair stereochemical activity, induced by the interaction between bismuth and oxygen atoms, influences the birefringence and nonlinear optical responses of different phases of BIBO. For birefringence, a coplanar arrangement of BO₃ groups is conducive to the realization of large birefringence; meanwhile, if the density of BO₃ in the unit cell is high, BO₃ plays a decisive role in large birefringence in BIBO; if the density of BO₃ is low, the effect of the bismuth oxygen polyhedra with lone pairs of electrons will lead to large birefringence in BIBO. Regarding the SHG response, bismuth oxygen polyhedra are responsible for large SHG responses, boron–oxygen groups made a small contribution. Moreover, by comparing the optical properties of BiB₂O₄F and BiB₂O₄(OH), it is also found that the lone pair stereochemical activity can be influenced by other ions such as fluorine in addition to oxygen. A comprehensive understanding is important to elucidate the structure–property relationship and provide a general strategy for searching for new NLO materials.