Raman, Brillouin, and nuclear magnetic resonance spectroscopic studies on shocked borosilicate glass

Using Brillouin and Raman scattering and NMR techniques, we have investigated the elastic and structural properties of four post-shocked specimens of borosilicate glass, recovered from peak pressures of 19.8, 31.3, 40.3, and 49.1 GPa. The Raman spectra of shock-wave compressed borosilicate glass for peak pressures of 19.8 and 31.3 GPa show two new peaks at 606 cm−1 and near 1600 cm−1, while a peak at ∼923 cm−1 disappears in these glasses following shock-loading. The mode at 606 cm−1 is correlated with four-membered rings, composed of one BO4 and three SiO4 tetrahedra (a reedmergneritelike configuration). Modes near ∼1600 cm−1 are of uncertain origin, while that at 923 cm−1 may associated with silica tetrahedra with two nonbridging oxygens, although standard models of this type of glass suggest that total nonbridging oxygen contents should be low. The Raman spectra for the shocked samples at 40.3 and 49.1 GPa are similar to that of the unshocked sample, suggesting that much of the irreversible density and structural changes are recoverable following adiabatic decompression and thermal relaxation. This reversibility for the highest pressure shocked samples is in accord with the Brillouin results, which show an increase in the product of sound velocity and index of refraction at pressures up to 20 GPa. The Raman band initially at 450 cm−1, which corresponds to the bending vibration mode of the Si–O–Si, Si–O–B (with primarily six-membered rings in the network) reaches a maximum frequency of 470 cm−1 and narrowing at a peak shock pressure of 31.3 GPa, and then also decreases to its initial values for samples shocked at 40.3 and 49.1 GPa. This shift toward higher frequency under shock-wave compression indicates the average Si–O–Si, Si-O-B angles decrease with pressure. The narrowing of this band suggests a narrower distribution of Si–O–Si angles in the shocked samples for peak pressures of 19.8 and 31.3 GPa. 11B NMR spectra for all four shocked glasses are similar, and indicate ratios of BO3 to BO4 that are not greatly changed from the starting material. However, changes in peak shapes suggest significant changes in the connectivity of the B and Si components of the network, with more silicon neighbors surrounding BO4 tetrahedra in the shocked glasses, and a modest increase in the number of nonring related BO3 groups following shock-loading. Thus, the irreversible effects of shock-loading appear to be to generate smaller rings of tetrahedra (hence decreasing the average T–O–T bond angle), and to increase the average number of neighbors of Si around boron tetrahedra.