Knowledge Bank

(A) The Raman and i.r. spectra of the newly synthesized second harmonic generating crystal $\beta$-barium metaborate in the low temperature phase are presented with a complete assignment of the external and internal vibrational modes. Emphasis is put on the discussion of the site group effect, the isotope effect and the dynamic coupling between the external and the internal motion of the metaborate ring and the barium ion. Force constants are also calculated with the prediction that there is one internal vibrational mode of the metaborate ring at a frequency of $177cm-1$ which is of the order of the frequencies of the lattice modes. (B) The Raman spectra of beta barium metaborate crystal are presented in the temperature range from $25\circ C$ to $400\circ C$. The spectra reveal that, as the temperature rises, the frequencies of the internal modes are unaffected. However, those of the lattice modes decrease slightly by about $3cm-1$. Results also show that the phase transition between the low and high temperature phases which are known in the literature occurs over a wide temperature range with no significant changes spectroscopically except the enhancement of the intensity of the translational mode of barium ions at $86cm-1$ as the temperature rises. Most intensity enhanced is of E symmetry. The enhancement is most drastic around $217\circ C$ and $317\circ C$. It is interpreted that the phase transition is mainly due to the rearrangement of the barium ions in the crystal. As the temperature rises, more barium ions are shifted to the sites of $C3$ and $D3$ symmetries. The role of the barium ion is also discussed along with its relationship with the second harmonic generating property of the bete barium metaborate crystal.