Quench Rate Studies of Aluminum Coordination and Oxygen Speciation in Calcium Aluminosilicate Glasses: Implications for Temperature Effects on the Structure of Aluminosilicate Melts

The atomic-scale structure of aluminosilicate glasses and melts is subject to temperature-induced rearrangement, which in turn controls variations in measurable thermodynamic properties. In order to investigate the effect of temperature on the structure of calcium aluminosilicate melts, we have used Al-27 MAS NMR, Al-27 3QMAS NMR, and O-17 MAS NMR to study fast- and slow-quenched calcium aluminosilicate glasses. Our previous work using O-17 3QMAS NMR to study lithium and sodium aluminosilicate glasses demonstrates disordering of bridging oxygen species with increasing sample fictive temperature, indicating temperature-induced rearrangement of framework units in the melt. Simple thermodynamic calculations using these results illustrate that while these structural changes may account for a portion of the experimentally-determined heat capacity of the melt, other modifications must occur to produce the observed temperature dependence of this property (Dubinsky and Stebbins 2006). The new data presented here allow us to observe changes in four-and five-coordinated aluminum and bridging and non- bridging oxygen (NBO) populations with fictive temperature in two calcium aluminosilicate glass compositions (CASx.y, where x=mol% Al2O3 and y=mol% SiO2) prepared by slow- and fast-quenching. We find that in CAS25.50 glasses, the percentage of five-coordinated Al per total Al increases from 7.6±1.9 to 9.4±2.4 and the percentage of NBO per total oxygen increases from 7.2±1.8 to 8.9±2.2 over a 150 to 200 degree fictive temperature interval. In CAS10.60 glasses representing a similar fictive temperature interval, we find that the percentage of five-coordinated Al per total Al increases from 4.0±1.0 to 4.9±1.2 and the percentage of non-bridging oxygen (NBO) per total oxygen increases from 22.8±2.3 to 23.0±2.3. Uncertainties in fitting procedures producing overlap in quantification of species in fast- and slow-quenched samples do not preclude the conclusion that robust changes in their relative proportions are visible in NMR spectra. Normalizing for sample composition, in both CAS25.50 and CAS10.60 glasses the increase in NBO over the studied fictive temperature interval exceeds the increase in five-coordinated Al, indicating that formation of these species is not necessarily coupled in a simple stoichiometric ratio. These results signify that complex structural changes involving high-coordinated Al and multiple high-coordinated oxygen species must be included in consideration of temperature effects on aluminosilicate melt structure.