An outstanding second-harmonic generation material BiB₂O₄F: exploiting the electron-withdrawing ability of fluorine

It is well known that the incorporation of fluorine usually blue-shifts the absorption edge of a crystal due to its large electronegativity. Herein, we validate the new functionality of fluorine that leads to the strong second harmonic generation (SHG) effect in a unique nonlinear optical (NLO) bismuth borate, BiB₂O₄F, where F coordinates to both B³⁺ and Bi³⁺ cations. The F-incorporation not only forms a more asymmetric unit [BO₃F] compared to the almost optical isotropic [BO₄], but also withdraws the non-bonding electrons of Bi³⁺, increasing its local polarization; thus the total SHG efficiency is increased significantly. Indeed, experimental measurements and first-principles calculations both support our anticipation that BiB₂O₄F is a phase-matchable NLO material with an exceptionally large SHG response. In order to further consolidate the functionality of fluorine, a comparison study with the structurally similar compound BiB₂O₄(OH) was performed.