Info

Fig. 11.18. On the left, the large figure shows the temperature dependence of the frequency of the Raman-active modes at (from top to bottom) (a) 1575 cm-1 [//g(8)], (b) 1467 cm-1 [Ag(2)], (c) 772 cm"1 [tfs(4)], (d) 707 cm"1 [Hg(3)], and (e) 496 cm"1 Mg(l)], respectively. The top panel on the left shows the temperature dependence of the 1467 cm'1 [Ag(2)] mode near the fee ->• sc transition, indicating a discontinuous transition with a width for the transition of ~1 K [11.44]. The figure on the right shows the temperature dependence of the linewidths of the observed Raman-active modes at (from top to bottom) 1575, 1467, 772, 707, and 496 cm"1, respectively. For both figures on the left (a) and right (b), the zero point on the vertical scale is indicated for each curve by the thick tick marks. (The solid lines are a guide to the eye.) [11.44].

fee sc transition (see §14.15(b)) are consistent with the volume decrease that is observed [11.44], A decrease in linewidth was reported for five of the Raman-active modes at the fcc-sc orientational phase transition (see Fig. 11.18(b)), with large decreases (factors of 3-4) in linewidth observed in the low-temperature phase for the Hg(3), Hg(4), and Hg(8) modes [11.44]. These large changes in linewidth were identified with enhanced vibrational-rotational coupling in the fee phase. Discontinuous changes in the intensity of the Raman lines at Tm were also reported [11.57],

Although the main effect of temperature variation on the Raman spectra occurs at Tox, significant changes in line intensities were observed upon lowering the temperature from 300 K down to low temperatures (10 K) [11.57], Mode splittings of some of the Raman-active modes were observed as well as the appearance of combination modes (see §11.5.3) [11.57]. Both the mode frequencies and linewidths of specific Raman features show lit-

tie temperature dependence in the sc phase for temperatures below T01 [11.44], although the increased intensities at low temperature bring out certain features in the Raman spectra that contribute to the background scattering near T01 [11.57], To illustrate the type of mode splittings that are observed in the Raman spectrum, we show the effect of temperature on the Ag(l) mode in Fig. 11.19. In this figure we see an asymmetric line at 300 K (above TQl), which shows well-resolved peaks at 10 K that appear to grow out of the scattering intensity in the low-frequency wing of the 300 K spectrum. Since crystal field splitting would not be expected above Tm for an Ag nondegenerate mode and the observed splittings below T0l are too large to be associated with an isotope effect, it is suggestive to identify the line asymmetry above Tm with local intermolecular orientational correlations, associated with fast ratcheting motion above T01 (see §7.1.3). At 10 K the local intermolecular orientational correlations become well defined and much stronger, giving rise to resolved Raman lines. The number of resolved Raman lines (three) in the 10 K spectrum is also suggestive of a local intermolecular correlation mechanism, since conventional crystal field splittings would give rise to only two mode frequencies because

Fig. 11.19. Raman spectra of the Ag(l) breathing mode of a C^o single crystal at (a) 300 K, (b) 10 K for the parallel scattering geometry, and (c) 10 K for the perpendicular scattering geometry. The solid lines are a Lorentzian fit to the data taken at

x(yz)x (c) innjfiiiuflif^

Was this article helpful?

0 0

Post a comment