AB-INITIO CALCULATIONS ON NORMAL-MODE VIBRATIONS AND THE RAMAN AND IR-SPECTRA OF THE [B3O6](3-) METABORATE RING

Normal vibrational modes of the [B3O6](3-) metaborate ring are obtaine d at the Hartree-Fock level by ab initio calculations. The Raman and I R spectra are predicted, and the former are compared with available ex perimental results of molten crystal metaborates that contain [B3O6](3 -) rings, i.e., beta-BaB2O4 and CsBO2 crystals. The internal vibration s of the [B3O6](3-) ring are dominant in the whole crystal lattice vib rations of these nonlinear optical crystals. Also, the [B3O6](3-) ring accounts for the IR transparency cut-off in such crystals. It is dedu ced from the predicted IR spectrum of a [B3O6](3-) ring that the trans parency cut-off frequency of the beta-BaB2O4 crystal on the IR side is below 4000 cm(-1) (>2.5 mu m), which agrees well with the measured da ta. This study is also useful for understanding the vibrational contri butions to nonlinear optical polarizabilities.