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

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 alpha, beta, gamma, delta, and epsilon-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.