A Theoretical Perspective to Study the Optical Properties of Tetrafluoroborates

In this study, a series of tetrafluoroborates with non-π-conjugated [BF₄ ] tetrahedra are investigated systematically by first-principles calculations. Theoretical studies demonstrate that tetrafluoroborates with alkali and/or alkaline-earth metals are more favorable for deep-ultraviolet transmission and are comparable to the classical deep-ultraviolet (deep-UV) material, MgF₂ . Furthermore, bandgap decrease with the increasing of ionic radii in alkali and/or alkaline-earth metals. Introducing highly polarizable cations with d₁₀ -configuration or cations with lone pair electrons into the structure will decrease the bandgaps. The birefringence and second harmonic generation effects are not large enough in tetrafluoroborates because polarizability anisotropy and hyperpolarizability in non-π-conjugated [BF₄ ] tetrahedra are much smaller than those in π-conjugated groups. However, the second harmonic generation effect for [BF₄ ] tetrahedra has a higher contribution in comparison with that due to birefringence. To effectively synthesize the borate fluorides or fluorooxoborates in the deep-UV region, raw materials with B-F bonds are preferred.