Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 196

In IR spectroscopy the sample is illuminated by radiation from an IR source [11, 12]. The incident photons are absorbed if there is a change in the induced dipole moment of the bonds undergoing the vibration. This is due to the nonuniform distribution of charge along the bond. The IR radiation is measured as it is passed through (absorbed) or reflected by the sample. In general, molecules or molecular groups that have strong changes in their dipole moments (polar molecules and asymmetric vibrations) usually have strong IR spectra. Some vibrations can be observed by both IR and Raman spectroscopy whereas others may only be observed by one or the other and hence which technique is employed depends somewhat upon the types of vibrations that will be investigated.

IR spectroscopy is widely used to investigate water [13] and carbon dioxide in glasses [14], boron (B) coordination in borate glasses [15], and the structure of sol‐gel derived glasses [16] and glass thin films. Kamitsos [12] has recently reviewed the application of IR spectroscopy to studies of glasses. The technique investigates the absorption or reflectance of IR radiation from the far (~10–400 cm⁻¹) to mid‐IR regions (~400–5000 cm⁻¹). The samples can be powders (mg), glass chips (mm), or preferably glass chips where the surface of the chip has been polished. Older transmission studies often used glass powders mixed with some sort of matrix material (usually an alkali halide).

Studies on volatiles are carried out with IR absorbance techniques whereas IR reflectance methods are the most common for structural studies of glasses (especially borates) because in this case one does not need to correct for a number of spectral aberrations caused by a variety of sources such as, for example, variations in sample thickness. Reflectance spectra are transformed with the Kramers–Krönig (KK) transformation or via dispersion analysis to provide the optical and dielectric properties of the glass (cf. [12]). As in Raman spectroscopy, the observed peaks are generally characteristic of specific vibrational motions and molecular groups. In borate glasses (Figure 7) peaks at 800–1150 cm⁻¹ are, for instance, due to B─O stretching vibrations of BO₄ tetrahedra whereas the bands at 1150–1550 cm⁻¹ are indicative of stretching vibrations of B─O bonds in triangular borate units. The progressive changes in the 800–1550 cm⁻¹ range observed in alkaline earth borates thus indicate a change in the B coordination from three to four when bond lengths and bond strengths change.

Figure 7 Band assignments in infrared absorption spectra of borate glasses obtained by Kramers–Krönig analyses of reflectance data, and effect on the absorption of the nature and concentration of (a) 33 mol % and (b) 45 mol % alkaline earth cation.

Source: After [12].