DFT-based ab initio study of dielectric and optical properties of bulk Li2B3O4F3 and Li2B6O9F2

► Theoretical and experimental studies of Li2B3O4F3 and Li2B6O9F2 are performed. ► The polarization of Li2B6O9F2 is measured experimentally and computed. ► Li2B6O9F2 was found to be a pyroelectric but not a ferroelectric material. ► Li2B6O9F2 has a higher nonlinear susceptibility than Li2B3O4F3. ► Theoretical nonlinear optical properties of Li2B6O9F2 are close to those of LiB3O5.