Vibrational dynamics in isotopically substituted vitreous GeO2

Abstract

We report the polarized Raman spectra of vitreous Ge ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}_2^{}{}_{}{}^{}$, Ge ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}_2^{}{}_{}{}^{}$, ${}_{}{}^{70}{}_{}{}^{}\mathrm{Ge}$ ${\mathrm{O}}_2$, and ${}_{}{}^{74}{}_{}{}^{}\mathrm{Ge}$ ${\mathrm{O}}_2$. This yields the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$→${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{70}{}_{}{}^{}\mathrm{Ge}$→${}_{}{}^{74}{}_{}{}^{}\mathrm{Ge}$ isotopic shifts for nearly all vibrational modes of the pure glassy material. The shifts of the broad high-frequency (infrared-active) modes are as predicted by a nearest-neighbor central-force ideal continuous—random-network model. The shift of the broad dominant Raman line indicates a small but significant dependence on the Ge mass, and this suggests an effect of disorder not included in the central-force theory. The narrow "defect" line at 530 ${\mathrm{cm}}^{-1\mathrm{}}$ appears to be all oxygen motion, and is tentatively identified with a regular ring of bonds. The narrow line at 345 ${\mathrm{cm}}^{-1\mathrm{}}$ is unique in that it exhibits very little oxygen shift; it seems to consist largely of Ge motion, for which we have no firm explanation.