Nature 292, 140 - 141 (09 July 1981); doi:10.1038/292140a0

Raman investigation of ring configurations in vitreous silica

Shiv K. Sharma^*, Joseph F. Mammone^† & Malcolm F. Nicol^†

^*Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA
^†Department of Chemistry, University of California, Los Angeles, California 90024, USA
^†Present address: Textile Fibers Department, Experimental Station, E. I. Du Pont de Nemours apd Co., Wilmington, Delaware 19898, USA.

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 observations⁵⁻¹¹ 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 invoked₅₋₉ 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, -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.

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