Ab Initio calculations on normal mode vibrations and The raman and IR spectra of the [B3O6]3- metaborate ring

Abstract

Normal vibrational modes of the [B3O6]3- metaborate ring are obtained at the Hartree-Fock level by ab initio calculations. The Raman and IR spectra are predicted, and the former are compared with available experimental results of molten crystal metaborates that contain [B3O6]3- rings, i.e., β-BaB2O4 and CsBO2 crystals. The internal vibrations of the [B3O6]3- ring are dominant in the whole crystal lattice vibrations of these nonlinear optical crystals. Also, the [B3O6]3- ring accounts for the IR transparency cut-off in such crystals. It is deduced from the predicted IR spectrum of a [B3O6]3- ring that the transparency cut-off frequency of the β-BaB2O4 crystal on the IR side is below 4000 cm-1 (>2.5 μm), which agrees well with the measured data. This study is also useful for understanding the vibrational contributions to nonlinear optical polarizabilities.