Spectral Resolution and Raman Q3 and Q2 cross sections in ~40 mol% Na2O glasses
Introduction
Insights into the short-range structure of silicate glasses and melts have been obtained primarily from, 29Si and 17O Nuclear Magnetic Resonance (NMR), Raman, and more recently, O 1s X-ray Photoelectron Spectroscopy (XPS) (e.g., Hehlen et al., 2017; Nesbitt et al., 2017a, Nesbitt et al., 2017b, Nesbitt et al., 2017c, Nesbitt et al., 2017d, Nesbitt et al., 2015a, Nesbitt et al., 2015b; Sawyer et al., 2015, Sawyer et al., 2012; Stebbins and Xue, 2014; Nesbitt and Bancroft, 2014; Nesbitt et al., 2011; Bancroft et al., 2009; Lee and Stebbins, 2009; Dalby et al., 2007; Neuville et al., 2014; Neuville, 2006; Xue et al., 1994; Mysen and Frantz, 1993; Maekawa et al., 1991; Stebbins, 1988, Stebbins, 1987; McMillan, 1984; Matson et al., 1983; Furukawa et al., 1981; Bruckner et al., 1980; Virgo et al., 1980; Brawer and White, 1975). These studies have sometimes allowed quantification of the five tetrahedrally coordinated Si species (Q species) in alkali silicate melts and glasses where each Q species hosts a different number of ‘Bridging Oxygen’ atoms (BOs) and ‘Non-Bridging Oxygen’ (NBO). The Si tetrahedra are referred to as Qn species, where the ‘n’ represents the number of BO's associated with the individual tetrahedra.
The number and types of Qn species strongly affect the chemical and physical properties of melts and glasses (e.g., Le Losq and Neuville, 2017) and the mechanisms of melting (e.g., Nesbitt et al., 2017d, Nesbitt et al., 2015a; Mysen and Richet, 2019; Richet et al., 1998, Richet et al., 1996, Richet et al., 1994), as well as reaction mechanisms (Nesbitt et al., 2020, Nesbitt et al., 2015a; Magnien et al., 2008). There is, therefore, good reason to obtain accurate Qn abundances in glasses and melts. In Raman spectra the abundance of such species is often estimated by comparing relative peak intensities or areas of the vibrational bands associated with the Qn species (i.e., Q4, Q3, Q2, Q1 and Q0). Furthermore, these intensities are often obtained from curve fits of the spectral envelope (cf., O'Shaughnessy et al., 2020). However, the area of the fitted component is not necessarily directly proportional to the actual fraction of a given structural species. This is because scattering cross sections for different Q species are expected to be affected to some degree by the short-range order (SRO) and the structure itself.
Previously, estimation of Raman cross-sections and quantitative Qn concentrations have been determined by calibration using the Q species numbers obtained by other methods (e.g., NMR spectroscopy) cf., Mysen and Richet, 2019. A problem arises, however, if the spectral envelope in the Raman spectra (~900–1300 cm−1) from which quantitative Qn numbers are being determined, contains peaks that are not due to Qn vibrations. This is the case for previous studies. Fits to the high frequency envelope always require extra peaks to be added to the fit (there are always more peaks than can be assigned to Qn vibrations), see below (cf., Henderson and Stebbins, 2021).
The objectives of the study are to establish the abundances of Q species in ~40 mol% Na2O glasses and to determine accurate Raman cross sections for the Q3 and Q2 species in these same glasses. Quantitative estimates of the Q species abundances derived from Raman spectra are contentious due to the difficulty of interpreting the spectra, as noted above (e.g., Nesbitt et al., 2017c; Sawyer et al., 2015; Malfait, 2015; Nesbitt et al., 2015b; Nesbitt et al., 2011). There is, however, good reason to quantify Raman spectra because it has superior resolution relative to other spectroscopic techniques (see below). If accurate Q3 and Q2 cross sections can be established for Raman spectra, it will be possible to obtain highly accurate estimates of Q species in glasses and more importantly, in high temperature silicate melts where most other spectroscopic techniques have very low resolution or cannot be used.
We undertake a detailed analysis of Q species abundances in glasses containing ~40 mol% Na2O, which necessarily includes evaluation of the properties of the Q3 line shape. As background, McMillan et al. (1992) argued that the Q3 line shapes was asymmetric and dependent on the local environment. The implication is that the asymmetry changes as the local environment changes. Nesbitt et al. (2017a), Bancroft et al. (2018), Nesbitt et al. (2019) and O'Shaughnessy et al. (2020) investigated line shapes using a variety of techniques and concluded that indeed the Q3 Raman signal was asymmetric and that its shape was a function of melt composition. With this aspect in mind, the Q2 and Q3 spectral intensities (peak areas) are evaluated for a highly resolved Raman spectrum containing 40.1 mol% Na2O glass. The implications of the results are then addressed.
Section snippets
Raman experimental considerations
The 40.1 mol% Na2O Raman spectrum used in this study was originally published by Neuville (2006) and he provides details of experimental methods. These are repeated here for the reader's convenience. A T64000 Jobin-Yvon confocal micro Raman spectrometer equipped with a CCD detector was used to collect data. Sample excitation was by a 514.532 nm line of a Coherent 70-C5 Ar+ laser operating at 2.8 W at the sample surface, resulting in a signal-to-noise ratio of 80/1. Integration time was 60 s.
Measurement of resolution
The comparative resolution of Raman, 29Si MAS NMR, 17O NMR and O 1s XPS spectroscopic techniques has not been evaluated previously but is required if uncertainties associated with reported Si and O species abundances are to be established across techniques. Resolution usually relates to FWHM or ‘linewidth’ of individual peaks relative to peak separation. This parameter is inevitably dependent on the peak shape (e.g., %Lorentzian; Hesse et al., 2007) used to fit spectral lines. Where two peaks
Q species abundances from 29Si NMR spectra
Four 29Si NMR studies of ~40 mol% Na2O glass (Fig. 1b) have been published (Nesbitt et al., 2011; Zhang et al., 1996; Maekawa et al., 1991; Stebbins, 1987) and their reported Q4, Q3, Q2 and Q1 abundances are provided in Table 1. Q0 species were not detected. The best resolved and best quality spectrum is that of Stebbins (1987) for the 41.1 mol% Na2O glass. He measured the Q4 species abundance using the more accurate 29Si Static NMR technique and reported a KD value of 0.011 (±0.005) for the
Establishing line shapes and widths
Ambiguities concerning interpretation of Raman spectra (noted above) have limited the use of the technique to obtain accurate Q species abundances (Bancroft et al., 2018; Neuville et al., 2014; Neuville, 2006; Mysen and Frantz, 1992, Mysen and Frantz, 1993; McMillan et al., 1992; Furukawa et al., 1981; Brawer and White, 1975). The Raman spectrum of the 40.1 mol% Na2O glass spectrum shown in Fig. 2a is composed of two well resolved bands centered at ~950 cm−1 and ~1100 cm−1. To establish line
Raman cross sections of Q2 and Q3 species
The spectral intensity of a Qn species (IQn) is the product of its cross-section (σQn) and mole fraction (XQn):
The Q3 and Q2 Raman spectral intensities are respectively 65.2 and 32.2 mol% for the 40.1 mol% Na2O glass (Table 1, Raman Intensities) whereas the best estimates of Q3 and Q2 abundances evaluated from the 29Si NMR results are respectively 68.8 and 28.8 mol% (Table 1). The Q3 cross section consequently is 0.95 (i.e., 65.2/68.8) and the Q2 cross section is 1.12 (i.e.,
Conclusions
The Q species abundances of four independent 29Si NMR studies of ~40 mol% Na2O glasses are in accord, as illustrated in Table 1. These results have been used to evaluate, for the first time, accurate Raman Q2 and Q3 cross sections in a glass containing 40.1 mol% Na2O. The asymmetric nature of the Q3 band is clearly apparent and must be addressed where Raman spectra are to be quantified. The results of Sen and Youngman (2003) and Olivier et al. (2001) demonstrate that Q species peaks (Q3 and Q4)
Declaration of Competing Interest
None.
Acknowledgements
The authors thank their respective Universities and departments for logistical support during the conduct of this research. GSH acknowledges funding through a NSERC discovery grant.
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