The dependence of crystal structure and ultraviolet nonlinear optical property of KBe<sub>2</sub>BO<sub>3</sub>F<sub>2</sub> by density functional theory and π-conjugate confinement

Abstract

<p indent="0mm">KBe₂BO₃F₂ (KBBF) is currently the only commercialized deep-ultraviolet (DUV) nonlinear optical (NLO) crystal, and its crystal structure is one of the most-utilized templates for the rational design; therefore, the understanding and elaboration of its structure–property relationship has an important significance. Herein, on the basis of density-functional theory (DFT), the bonding mode and the electronic structure of the KBBF are deeply investigated, and the contributions of the component groups, <italic>i</italic>.<italic>e</italic>., the BO₃ π-conjugated group and the BeO₃F non-π-conjugated group, to the DUV–NLO properties are explored and explained. The “π-conjugated confinement” feature of the KBBF structure allows it to achieve the highly desired balance between the wide band-gap and the large birefringence, and eventually enables KBBF to achieve the second harmonic generation in the high-energy DUV-spectral region. These understanding and elaboration of the “π-conjugated confinement” may provide useful insights for the future design of the DUV–NLO materials.