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

Abstract

Raman spectra of solid boron triiodide, BI 3 , at ambient temperature and at pressures up to 5 GPa are reported. Because of drastic but reversible colour changes to the samples and subsequent absorption, measurements at higher pressures were not possible. Four fundamental peaks, assigned as translational, librational, bending and stretching modes, were tracked as a function of pressure. The spectra evolve smoothly and there is no evidence of any structural phase transitions or major molecular distortions in this pressure range. All modes show increases in wavenumber with pressure, with that for the two lattice modes being most pronounced. The initially 'soft' crystal at ambient pressure stiffens appreciably, with an approximately fourfold increase in intermolecular bonding at 5 GPa.

Raman spectra of molecular crystals at high pressures: VIII—Boron tribromide

“…The two peaks observed in the intramolecular region correspond to 1 , the symmetric stretch, and 4 , the degenerate in-plane bend. As for boron triiodide, 1 2 is Raman inactive and 3 , although nominally active, is known to have very low Raman intensity. The small isotopic splittings, which have been observed for 1 in samples at low temperatures, 7,10 cannot be resolved in these experiments at ambient temperature.…”

“…In other words, the initially soft crystal stiffens appreciably under external pressure, as was also found for boron triiodide. 1 A comparison of the Raman spectra of these two crystals at high pressures is slightly complicated by the fact that, for BI 3 , measurements were limited to below 5 GPa because of colour changes and resultant absorption effects. 1 As a result, the quadratic dependence of wavenumbers on pressure observed for BBr 3 , with values appearing to approach asymptotic limits, was not found for BI 3 .…”

“…In a recent paper, 1 we reported on the Raman spectra of the molecular crystal of boron triiodide at ambient temperature and at pressures up to 5 GPa. Reversible colour changes and resulting absorption effects prevented measurements at higher pressures.…”

“…The correlation diagram linking the normal-mode representations for the molecular, site and unit cell symmetries has been presented in an earlier paper. 1 It is likely that the crystal formed by applying pressure to the liquid at ambient temperature has the same structure, although some exceptions to this rule are known. 6 The predictions for the Raman spectra are as follows: three of the four internal modes ( 1 , 3 and 4 ), the librations (L z and L xy ) and the optical translation (T xy ) should each produce one Raman component, whereas the out-ofplane bend ( 2 ) and the optical translation (T z ) are Raman inactive.…”

Pressure-Induced Metallization of Molecular Crystal BI3

“…Previous Raman spectra of BI 3 under high pressure reported that there was no evidence of any structural phase transition or notable molecular distortion up to 5 GPa. 5) Our recent results by X-ray diffraction study up to 19 GPa revealed a structural phase transition at 6.9 GPa to a new phase, in which the molecule seemed to dissociate. 6) By applying pressure, the transparent BI 3 crystal gradually became into opaque.…”