A new approach to determine and quantify structural units in silicate glasses using micro-reflectance Fourier-Transform infrared spectroscopy

Eight silicate unit vibrational modes were identified in a suite of PbO-SiO₂ glasses using microreflectance Fourier Transform infrared (μR-FTIR) spectra that were transformed using the Kramers-Kronig relation. The transformed FTIR spectra, in the 800-1200 cm^-1 range, were deconvolved systematically into eight Voigt-shaped bands at centers that were predicted from the second derivative of the spectra. The area of the eight bands varied as a function of SiO₂ content, and these trends were combined with theoretical constraints to identify and assign the bands to seven provisional silicate units: SiO₄^4- (830 and 860 cm^-1), Si₂O₇^6- (900 cm^-1), Si₆O₁₈^12- (950 cm^-1), Si₂O6^4- (980 cm^-1), Si₄O₁₁^6- (1010 cm^-1), Si₂O₅^2- (1050 cm^-1), and SiO₂ (1100 cm^-1). The provisional units were then grouped according to their NBO/T values: NBO/T = 4 (SiO₄ ^4-), NBO/T = 3 (Si₂O₇ ^6-), NBO/T = 2 (S_i6O₁₈ ^12- and Si₂O₆^4-), NBO/T = 1 (Si₄O₁₁^6- and Si₂O₅^2-) and NBO/T = 0 (SiO₂). The derived quantities of each NBO/T unit compare favorably with nuclear magnetic resonance data for PbO-SiO₂ glasses reported in the literature. This new approach for determining glass structure is advantageous because it may be performed on small Fe-bearing samples with minimal preparation, and analyses are rapid and relatively inexpensive.