Investigaton of phase separation by Raman spectroscopy

Abstract

Raman spectra of silica and borate glasses characteristically consist of a few rather broad bands, the frequencies of which are related to the network-forming units - silica tetrahedra, boron triangles, boron tetrahedra, etc. - and the extent to which these units are polymerized. Heat treatment that produces crystallization develops new sharp Raman bands which increase in intensity with time of heat treatment and which measure the evolution of the crystalline phases. Raman spectra of heat-treated glasses for which heat treatment produces glass-glass phase separation are in most cases identical to the original glass. Examination of the Raman spectra of phase-separated alkali borate and alkali silicate glasses leads to the conclusion that the melts from which these glasses are prepared are already clustered on an atomic scale so that they contain all of the network-forming linkages that will be segregated into thermodynamically distinct phases after heat treatment. The minimum size of the clusters is difficult to estimate but deductions from the spectra of germano-silicate glasses suggest it must be on the order of 5–10 tetrahedra.