Lattice vibrations of boron trihalide crystals

Abstract

The infrared and Raman spectra of polycrystalline samples of Be1 3 , BBr 3 , and BI3 at 80 and 18 K have been recorded. In the intramolecular region, isotopic and crystalline field splittings are observed in substantial agreement with earlier work. In the lattice region, no far-infrared absorption has been found, in accordance with predictions from a group theoretical analysis based on the known structures of these isomorphous crystals. The three observed Raman lattice modes are assigned according to these procedures, and their frequencies are compared with those derived from calculations of the lattice statics and dynamics of these crystals. It is found that a four-parameter model using the rigid molecule approximation and simple pairwise interaction potentials between X-X and B-X atoms is capable of giving acceptable values of the zone center frequencies while simultaneously satisfying the static equilibrium conditions for these crystals.

Raman spectra of molecular crystals at high pressures: VII—Boron Triiodide

“…As shown in the correlation diagram (Fig. 2), 2 is Raman inactive and although 3 is nominally Raman active, it is known to be very weak 15 and could only be detected at low pressures. Of the three predicted lattice modes, only two were observed in a previous low-temperature Raman study, 15 probably because of an accidental near degeneracy.…”

“…This is based both on lattice dynamics calculations 15 and on the fact that for many molecular crystals, including some where deuterated samples have allowed unequivocal assignments of lattice modes, librations generally occur at higher wavenumbers than translations. The higher wavenumber peak is a libration, probably L xy rather than L z , since the latter is inactive for the free molecule, and so its intensity in the crystal is derived solely from intermolecular perturbations.…”

“…This corresponds to a fourfold strengthening of the intermolecular forces involved (primarily between neighbouring iodine atoms 15 ), if harmonic interactions are assumed. As a result, the separation between these two modes increases from 14 cm 1 at 0 GPa to 23 cm 1 at 5 GPa.…”

“…This interpretation is supported by a comparison with the other boron trihalides. 15 The separation between these modes is 5.5 cm 1 for BCl 3 and 3.5 cm 1 for BBr 3 and so is expected to be even smaller for BI 3 .…”

“…A Raman study of the crystal at 77 K was published by Clark and Mitchell, but only internal modes A. Anderson and L. Lettress were observed. 14 Finally, Raman and infrared spectra of crystals at about 20 K have been reported by Binbrek et al 15 Lattice modes were observed for the first time and their wavenumbers were compared with results from simple lattice dynamics calculations. To the best of our knowledge, no previous papers on the vibrational spectra of boron triiodide at elevated pressures have been published.…”