AUTHOR=Calahoo Courtney , Petrovic Jelena , Le Quyen Huyen , Werner-Zwanziger Ulrike , Zwanziger Josef , Wondraczek Lothar 

TITLE=Structural Relaxation in Polyanionic Sodium Fluorophosphate Glasses

JOURNAL=Frontiers in Materials

VOLUME=6

YEAR=2019

URL=https://www.frontiersin.org/journals/materials/articles/10.3389/fmats.2019.00165

DOI=10.3389/fmats.2019.00165

ISSN=2296-8016

ABSTRACT=<p>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-<italic>x</italic>)NaPO<sub>3</sub>-<italic>x</italic>AlF<sub>3</sub> 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, <italic>E</italic><sub>a</sub>, 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 <sup>31</sup>P, <sup>19</sup>F, and <sup>27</sup>Al NMR, we observe that the introduction of AlF<sub>3</sub> into the NaPO<sub>3</sub> network increases the <italic>average</italic> connectivity (i.e., the number of heteronuclear Al—O—P bonds), rationalizing the higher <italic>E</italic><sub>a</sub> determined from the DSC measurements. We find highly constrained regions of Al(OP)<sub>4</sub>F<sub>2</sub> with Al—F—Al cross-linking (high <italic>E</italic><sub>a</sub>) and, simultaneously, more flexible regions of phosphate chains containing P—F··(Na<sup>+</sup>)<sub><italic>n</italic></sub> bonds (low <italic>E</italic><sub>a</sub>); this results in a topologically and dynamically heterogeneous structure as evidenced by the increased variability in <italic>E</italic><sub>a</sub> with higher AlF<sub>3</sub> content. The decreasing size of the CRR reflects the increased heterogeneity: at low AlF<sub>3</sub> the CRR is large, while at high AlF<sub>3</sub> (high heterogeneity), the CRR is significantly smaller (by a factor of 10<sup>3</sup>). Finally, we relate the heterogeneity to other macroscopic properties, such as <italic>T</italic><sub>g</sub> and mechanical properties.</p>