Raman spectroscopic investigation of the structure of silicate glasses. I. The binary alkali silicates

The Raman spectra of some binary alkali silicate glasses and crystals have been measured and the results interpreted. The glasses are of composition M2O⋅xSiO2, 1⩽x⩽4 and M = Li, Na, K. There is a strong resemblance between glassy and crystalline spectra which is consistent with a considerable amount of structural disorder of the glass. The reason is that the disorder acts as a perturbation on the spectrum of an elementary vibrating unit from which the glass is largely constructed. As a result, the general nature of the glass structure may be determined. Furthermore, glasses of the same x but with different alkali have different amounts of short-range order. This is demonstrated by showing that the widths of certain Raman peaks increase with increasing disorder of the Si–O network of the glass. A strong argument is made against the microcrystallite theory of glass structure. For any composition x, glasses increase in disorder in the direction K → Na → Li. The crystallization rates increase in the same direction, just the opposite of what would be expected on the basis of a microcrystallite theory. Finally, evidence is given for the existence of microscopic concentration fluctuations in presumably single-phase disilicate glasses.