Structural Relaxation in Polyanionic Sodium Fluorophosphate Glasses

Abstract

Structural heterogeneity is a common feature of all glasses, however, little is known about the underlying contributions of chemical fluctuations and modulations in free volume in concrete glass forming systems. In this investigation, we relate the dynamics of structural relaxation of (100-x)NaPO 3-xAlF 3 glasses to their heterogeneous structure as determined from multinuclear magnetic resonance spectroscopic analysis. For this, we evaluate differential scanning calorimetry (DSC) data using the integral isoconversional method to determine the variation in activation energy, E a , of the glass transition as a function of temperature and conversion progress. Specific heat measurements from DSC allow for the determination of the effective size of the cooperatively rearranging region (CRR). From 31 P, 19 F, and 27 Al NMR, we observe that the introduction of AlF 3 into the NaPO 3 network increases the average connectivity (i.e., the number of heteronuclear Al-O-P bonds), rationalizing the higher E a determined from the DSC measurements. We find highly constrained regions of Al(OP) 4 F 2 with Al-F-Al cross-linking (high E a) and, simultaneously, more flexible regions of phosphate chains containing P-F••(Na +) n bonds (low E a); this results in a topologically and dynamically heterogeneous structure as evidenced by the increased variability in E a with higher AlF 3 content. The decreasing size of the CRR reflects the increased heterogeneity: at low AlF 3 the CRR is large, while at high AlF 3 (high heterogeneity), the CRR is significantly smaller (by a factor of 10 3). Finally, we relate the heterogeneity to other macroscopic properties, such as T g and mechanical properties.

Optimization of electrical conductivity in the Na2O‐P2O5‐AlF3‐SO3 glass system

“…Previously published 19 F-NMR of similar Na 2 O-P 2 O 5 -AlF 3 (NAPF) glasses found three different fluorine species: nonbridging fluorine, Al-F••Na + (located at −170 ppm), fluoride bridges, Al-F-Al (−147 ppm), and covalent P-F bonds (−75 ppm). 8,10 The existence of aluminum-fluoride bonds in the current series was confirmed by Raman spectroscopic analysis, at 540 cm −1 (νAl(F,O) 6 ) and 630 cm −1 (ν as Al-F) in Figure 2. The current Raman results, combined with previous NMR data on NAPF glasses, and with other literature 8,10,11 indicate that aluminum links to phosphate units through P-O-Al linkages to form Al(OP) 4 F 2 octahedra.…”

“…8,10 The existence of aluminum-fluoride bonds in the current series was confirmed by Raman spectroscopic analysis, at 540 cm −1 (νAl(F,O) 6 ) and 630 cm −1 (ν as Al-F) in Figure 2. The current Raman results, combined with previous NMR data on NAPF glasses, and with other literature 8,10,11 indicate that aluminum links to phosphate units through P-O-Al linkages to form Al(OP) 4 F 2 octahedra. We observe in Figure 4 by the respective growth of the Q 1 Al and Q 2 Al Raman bands at 1068 and 1090 cm −1 ).…”

“…Additionally, we expect higher AlF 3 content to result in more Al-F-Al linkages (from previous 19 F-NMR). 8,10 As observed elsewhere, these strong, directional bonding interactions are responsible for the decrease in Poisson ratio, and for increasing glass transition temperature and Young's modulus. 4,12 In order to better separate the individual effects of AlF 3 and SO 3 addition on the NaPO 3 glass structure, comparisons of the appropriate Raman spectra are provided in Figure 5.…”

“…To understand the mechanism of electrical conductivity, the structure of NAPFS glasses was initially elucidated through a combination of previous Raman and 31 P-NMR spectroscopic observations (from Le et al 4 ), along with 19 F-NMR and 27 Al-NMR spectroscopy data (from Calahoo et al 8 ). In addition to the new Raman spectra presented here, these previous data 4 were re-evaluated and analyzed further by band deconvolution.…”

“…In our glasses with AlF 3 as the source of aluminum, the 27 Al-NMR spectra indicate that almost all aluminum atoms have sixfold coordination (even at high [Al/P] where fourfold Al is expected to form, fluorine content forces octahedral coordination). [8][9][10] Consequently, we expect all investigated compositions to have octahedral aluminum sites. Previously published 19 F-NMR of similar Na 2 O-P 2 O 5 -AlF 3 (NAPF) glasses found three different fluorine species: nonbridging fluorine, Al-F••Na + (located at −170 ppm), fluoride bridges, Al-F-Al (−147 ppm), and covalent P-F bonds (−75 ppm).…”