Raman investigation of ring configurations in vitreous silica

Abstract

Random network models of glass structures¹ provide satisfactory qualitative descriptions of many properties of glasses (see reviews in refs 2–4). However, to obtain good quantitative agreement between theoretical analyses and experimental observations^5–11 it is often necessary to assume that specific ring structures in the random networks have special importance. In the case of vitreous silica (v-SiO₂), distributions of loops of SiO₄ tetrahedra in the random network have been invoked_5–9 to match calculated and experimental X-ray radial distribution functions (RDF). High resolution X-ray photoelectron spectra¹² of v-SiO₂ and quartz also provide evidence for the occurrence of four-, six- and higher-membered rings of SiO₄ tetrahedra in v-SiO₂. Raman spectra are also sensitive to local microstructures in vitreous solids. We have therefore examined the question of rings in the structure of v-SiO₂ by comparing its Raman spectrum with spectra of crystalline silica polymorphs whose shortest loops contain four (coesite) and six (for example, α-quartz) tetrahedra. This comparison indicates that the sharp shoulder at 490 cm⁻¹ in the spectrum of v-SiO₂, previously attributed to a defect structure3 or a longitudinal optic mode¹⁴, can be assigned to four-membered ring structure. We discuss here possible basis for the stability of four-membered rings of SiO₄ tetrahedra in v-SiO₂ at ambient pressure. Possible reasons for the absence of the 490 cm⁻¹ band in the vibrational density-of-states derived for a random network model by Bell and co-workers^12,15 are discussed.