Structural Studies of NaPO₃–AlF₃ Glasses by High-Resolution Double-Resonance Nuclear Magnetic Resonance Spectroscopy

Abstract

The local structure of the model glasses (NaPO 3 ) 1−x -(AlF 3 ) x (0 ≤ x ≤ 0.4), prepared by standard melt-cooling, was extensively investigated by high-resolution solidstate nuclear magnetic resonance (NMR) including advanced double-resonance techniques. This glass system offers the opportunity of studying five different heteronuclear distance correlations (Na−F, Na−P, P−F, Al−F, and P−Al) by 10 distinct double-resonance experiments, involving all of the constituent elements present. 27 Al MAS-NMR data indicate that aluminum is predominantly six-coordinated. According to 27 Al{ 31 P} and 27 Al{ 19 F} rotational-echo double-resonance (REDOR) spectroscopic results, two to three Al−F and three to four Al−O−P linkages occur in these glasses, independent of composition x. 19 F MAS-NMR spectra show the presence of terminal P-bound and Al-bound fluorine species. A small amount of fluorine bridging to two aluminum octahedra, which could be assigned based on 19 F{ 27 Al} and 19 F{ 31 P} REDOR experiments, was also detected. 19 F{ 23 Na} REDOR experiments indicate that the Al-bound terminal F atoms interact significantly more strongly with sodium ions than the P-bonded terminal F atoms, which is consistent with local charge considerations. On the basis of the detailed quantitative dipole−dipole coupling information obtained, a comprehensive structural model for these glasses is presented.

Structural Relaxation in Polyanionic Sodium Fluorophosphate Glasses

“…Given the low intensities of Al [5] and Al[4] coordinations, integration was fixed to a chemical shift range based on the 60NaPO 3 -40AlF 3 sample, shown in Figure 5; our final values are given in Table 6. These values agree with the descriptions reported elsewhere on this same glass series: Bradtmüller et al (2018) demonstrate that at low AlF 3 content, all the aluminum is 6-fold coordinated and only at high amounts of AlF 3 (40 mol%) are small amounts of 5-fold (8%) and 4-fold (1%) aluminum present. Similarly, Brow (Brow, 1992) observed only one central peak at −10 ppm in the 27 Al NMR spectra of this glass series, without any evidence for Al [4] or Al [5].…”

“…Although the four peaks overlap, the area fractions show the expected monotonic exchange between the Q n species with and without aluminum neighbors. Moreover, the trends match those of Bradtmüller et al (2018) found for the same glass series, including the increase and then subsequent decrease of the Q 1 0Al species. The exact fitting 31 P NMR parameters can be found in Table 2.…”

“…The 19 F NMR spectra in The assignments are based on NMR studies of similar glass systems and double resonance NMR experiments, which showed a lack of Al-F-P bonds (Le et al, 2017;Bradtmüller et al, 2018). These assignments are validated further by calculated "islands" of Al, F, and Na exist; the chemical shifts found in these glasses correspond to crystals containing phosphate.…”

“…Given that aluminum is an intermediate glass former and similar literature (Le et al, 2017), we assign these new peaks to Q 1 1Al and Q 2 1Al, respectively. In this paper, we follow the terminology used by Bradtmüller et al (2018): Q n m notation is the typical Q n notation, with n being the number of homonuclear phosphate bonds and m being the number of aluminum neighbors, whereas in the P q m notation, q refers to number of total bridging oxygens (P or Al). For the 10 and 20 mol% AlF 3 , the peaks with one aluminum neighbor (Q 1 1Al and Q 2 1Al ) grow in at the expense of the Q 1 0Al and Q 2 0Al peaks, and commensurately with the amount of AlF 3 in the glass.…”

“…Here, we report on the calorimetric investigation of the structural relaxation in fluoride-phosphate mixed-anion glasses of the type (100-x)NaPO 3 -xAlF 3 (NAPF) using the integral isoconversional method. In those glasses, large amounts of aluminum fluoride can be dissolved (Brow, 1992;Zhang et al, 2007;Le et al, 2017;Bradtmüller et al, 2018). The simultaneous formation of strong Al-O-P bonds and weaker terminal Al-F or P-F bonds reported (Brow, 1992) makes the NAPF system an interesting candidate for studying structural heterogeneity; there are highly constrained, strongly bonded regions (bridging O or F) interspersed among more flexible, less-constrained regions (NBOs or terminal F).…”