Polarized, depolarized, isotropic, difference, and reduced Raman spectra were obtained from vitreous silica, irreversibly compacted at 600 °C and 50 kbar, to ambient densities as high as 2.73 g cm−3. The 60 cm−1 ‘‘Boson’’ peak; and, the intense optic, Si–O–Si bending peak at 440 cm−1 including its low‐frequency shoulders at ≊210–240 and ≊340 cm−1; were observed to move strongly upward in position, but at two different rates with density rise. The two peak frequencies behave experimentally like spherical transverse acoustic (TA) or shear (S), and spherical longitudinal acoustic (LA) or pressure (P) stress waves of an isotropic elastic solid. In contrast, downward frequency shifts were observed for the transverse optic (TO) 1060 cm−1 and longitudinal optic (LO) 1200 cm−1 modes in the Si–O stretching region, but at absolute rates equal, respectively, to those of the 60 and 440 cm−1 peak frequencies thus linking optic as well as acoustic modes to the elastic modulii.
The TO and LO splitting was shown from a phonon coupling model to result from very strong interaction between the optic modes and the low‐frequency S and P modes. The frequency decreases of the TO and LO modes indicate a decrease in the Si–O stretching force constants, related to a decrease in the mean Si–O–Si bridging angle, from ≊144° to ≊120°, inferred from frequency increases at 440 and ≊800 cm−1. A decrease in the mean O–2nd‐O distance also occurs, but severe distortion of the SiO4 tetrahedra does not seem probable to densities of 2.73 g cm−3. The dilatational P‐mode wavelength ≥5 Å corresponds to volume‐related distances, e.g., Si–2nd‐O, O–2nd‐O, or Si–2nd‐Si. The shear mode wavelength is larger (≊13 Å) and similar to the structural correlation length, estimated roughly from x‐ray data, where G(r)≊1. Both S and P modes involve small coherence volumes, and both may have negligible group velocities, i.e., standing waves. The low‐frequency S scattering is weakly to completely depolarized, but the higher‐frequency P scattering may be strongly polarized, falling near or just below (BeF2, B2O3, GeO2, SiO2, ZBLAL) the first very strong minimum in the Raman depolarization ratio, located above the S mode.
This new criterion locates the actual P mode of fused silica within the ≊200–350 cm−1 shoulder region. This region is intense in the density of states, determined by neutron inelastic scattering (NIS). S and P modes are resolved in the Raman spectra of several glasses, but their relation to the low‐order spherical acoustic stress waves of isotropic media has, heretofore, not been widely recognized.