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Structure of P2O5–SiO2 Pure Network Former Glasses Studied by Solid State NMR Spectroscopy C
- de Oliveira, Marcos, Aitken, Bruce, Eckert, Hellmut
- Journal of physical chemistry 2018 v.122 no.34 pp. 19807-19815
- Raman spectroscopy, glass transition, nuclear magnetic resonance spectroscopy, phosphates, phosphorus, phosphorus pentoxide, silica, silicon, solids, stable isotopes, temperature
- The structure of binary (SiO₂)₁₀₀₋ₓ–(P₂O₅)ₓ glasses has been investigated by Raman scattering, ²⁹Si and ³¹P magic angle spinning (MAS) as well as static ³¹P NMR spectroscopy. ²⁹Si chemical shift trends reflect the successive replacement of Si–O–Si by Si–O–P linkages as the compositional parameter x is increased. While ³¹P MAS NMR does not resolve separate phosphate species, the static ³¹P NMR lineshapes were successfully simulated by considering the effect of uncorrelated distribution functions of the chemical shift tensor components upon the line shape. On the basis of these simulations, which were also found to be consistent with the experimental ³¹P MAS NMR spectra, two distinct sites can be resolved: a dominant site characterized by an axially symmetric chemical shift tensor, assigned to P⁽³⁾ units, and (only in the case of the x = 25 and 30 glasses) a Gaussian component reflecting phosphate species interacting with five- and six-coordinated silicon species. For 0 ≤ x < 25, the decrease in average coordination number may provide the structural explanation for the strong decrease in the glass transition and liquidus temperatures over this composition range, whereas the subsequent increase in Tg at higher P₂O₅ contents is correlated with the appearance of the higher-coordinated silicon species. While these higher-coordinated silicon species occur within separate microdomains, ³¹P spin echo decay spectroscopy suggests that the majority of P atoms tend to be randomly distributed in space, consistent with a statistical P–O–P, Si–O–P, and Si–O–Si connectivity distribution.