Vibrational dynamics inSi30-substituted vitreous SiO2

We report the polarized Raman spectra of vitreous ${}_{}{}^{30}{}_{}{}^{}\mathrm{Si}$ ${\mathrm{O}}_2$ and compare the principal frequencies with those of vitreous ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ ${\mathrm{O}}_2$. This yields the ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$→${}_{}{}^{30}{}_{}{}^{}\mathrm{Si}$ isotopic shifts for nearly all vibrational modes of the pure glassy material. The shifts of the high-frequency (infrared-active) modes are as predicted by a nearest-neighbor central-force ideal continuous-random-network model originally due to Sen and Thorpe. The dominant Raman line shows a modest but significant dependence on the Si mass, and this points to an effect of disorder not included in the theory. The Raman-active "defect" lines exhibit essentially no Si mass dependence, which is consistent with their recent assignment to modes of highly ordered (planar) rings of bonds, and inconsistent with numerous other (broken-bond and wrong-bond) models.