Abstract
The birefringence of the new Ba2Na3[B3O6]2F (BNBF) (P63/m) and Ba7(BO3)4−yF2+3y (P63) phases has been investigated. Coefficients of the Sellmeier equations have been determined. Parallel [B3O6]3− rings in the Ba2Na3[B3O6]2F structure generate the major anisotropic polarizabilities and produce large birefringence Δn=0.1097−0.0428 from 350nm to 5μm, comparable to that of commercial birefringent crystal α-BaB2O4. A new flux for growing a high quality BNBF crystal has been proposed. In the measured wavelength range from 266nm to 1.7μm noncentrosymmetric phase Ba7(BO3)4−yF2+3y (y=0.49) exhibits low birefringence Δn=0.0111−0.0037 which can be accounted for non-coplanar arrangement of the anionic groups in the structure. Low birefringence prevents the use of this crystal for nonlinear optical applications, since phase-matching conditions for second harmonic generation can not be reached.
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