Searching for correlations between vibrational spectral features and structural parameters of silicate glass network

Abstract

Infrared (IR) and Raman spectroscopic features of silicate glasses are often interpreted based on the analogy with those of smaller molecules, molecular clusters, or crystalline counterparts; this study tests the accuracy and validity of these widely cited peak assignment schemes by comparing vibrational spectral features with bond parameters of the glass network created by molecular dynamics (MD) simulations. A series of sodium silicate glasses with compositions of [Na2O]x[Al2O3]2[SiO2]98−x with x = 7, 12, 17, and 22 were synthesized and analyzed with IR and Raman. A silica glass substrate and a crystalline quartz were also analyzed for comparison. Glass structures with the same compositions were generated with MD simulations using three types of potentials: fixed partial charge pairwise (Teter), partial diffuse charge potential (MGFF), and bond order‐based charge transfer potential (ReaxFF). The comparison of simulated and experimental IR spectra showed that, among these three potentials tested, ReaxFF reproduces the concentration dependence of spectral features closest to the experimentally observed trend. Thus, the bond length and angle distributions as well as Si–Qn species and ring size distributions of silica and sodium silicate glasses were obtained from ReaxFF‐MD simulations and further compared with the peak assignment or deconvolution schemes—which have been widely used since 1970s and 1980s—(a) correlation between the IR peak position in the Si–O stretch region (1050‐1120 cm−1) and the Si–O–Si bond angle; (b) deconvolution of the Raman bands in the Si–O stretch region with the Qn speciation; and (c) assignment of the Raman bands in the 420‐600 cm−1 region to the bending modes of (SiO)n rings with different sizes (typically, n = 3‐6). The comparisons showed that none of these widely used methods is congruent with the bond parameters or structures of silicate glass networks produced via ReaxFF‐MD simulations. This finding invokes that the adequacy of these spectral interpretation methods must be questioned. Alternative interpretations are proposed, which are to be tested independently in future studies.

Effects of Al:Si and (Al + Na):Si ratios on the properties of the international simple glass, part I: Physical properties

“…The decrease in υ Si‐O intensity is due to the reduction in the Si‐O bonds and the refractive index change as Al concentration increases 86 . Shifts towards lower wave numbers indicate longer Si‐O bond length distributions 87‐89 likely due to increases in Si‐O‐Al and/or NBOs associated with Si. The peak maximum of ISG‐A00N is shifted further toward the higher wave numbers of ISG‐C compared to ISG‐A00 by ~30 cm −1 which is likely caused by immiscible phase separation.…”

“…further toward the higher wave numbers of ISG-C compared to ISG-A00 by ~30 cm −1 which is likely caused by immiscible phase separation. Based on the blue shift of the υ Si-O peak, the separated phase is expected to have shorter Si-O bond length distributions, [87][88][89] that is, closer to a silica-like structure.…”

Chemical durability of borosilicate pharmaceutical glasses: Mixed alkaline earth effect with varying [MgO]/[CaO] ratio

“…33,34,45,46 We have shown that the main Si-O-Si stretch peak at ~1060 cm −1 can be related to the Si-O bond length distribution of the network in the case of silica and sodium silicate glasses. 14,47 Since the boron and aluminum contents are quite low here (see Table 1), we assume that the same could be applied to the glass series tested in this study qualitatively. Then, changes in the Si-O-Si stretch band, especially peak position and shape, could mean alteration of the glass structure in the corroded layer likely due to network re-arrangement or re-polymerization.…”

“…11,12 For high durability glasses, the characterization of the corrosion behavior is challenging due to the low concentrations of ions being released into solution. 13,14 Despite the recent progress in pharmaceutical glass packaging, further improvement of the properties of the glass is demanded to meet the requirements related to mechanical strength and chemical durability. 15 To improve the chemical durability and mechanical properties of pharmaceutical glasses, a chemical strengthening treatment has often been applied.…”

“…According to international standards, the concentrations of leached ions should not exceed limiting values, which are quite low, to ensure quality over the lifetime of the drug 11,12 . For high durability glasses, the characterization of the corrosion behavior is challenging due to the low concentrations of ions being released into solution 13,14 . Despite the recent progress in pharmaceutical glass packaging, further improvement of the properties of the glass is demanded to meet the requirements related to mechanical strength and chemical durability 15…”