Short- and medium-range order in silicate glasses and melts: Insights from Raman and NMR spectroscopy and effects on bulk melt properties

Abstract

The physical and thennodynamicproperties ofmagmas control the rate and style of a wide range of igneous processes. To a large extent, these properties are determined by the properties of the Silicate melt phase. Thus, Silicate melts play an essential role in the mass and heat transfer in the earth and terrestrial planets. For multi-component melts, the properties generally are not simple additive functions of the chemical composition,but strongly depend on the microscopic structure. This thesis focuses on the structureof silica glass and alkali Silicate glasses and melts. The results are then related to the bulk melt properties. The influence of the structure on the properties of alkali Silicate glasses was investigated by Si Single Quantum and Double Quantum MASNMRspectroscopy. The chemical shifts and the relative abundance of the ff species correlate with the cationic field strength (Z/r) of the network modifier. A correlation is observed between the glass transitiontemperature(Tg) and the inverse of the entropy of mixing of the differentff species, whichis explainedin the frameworkof the Adam-Gibbs relaxation theory. At high alkali content, up to 44% of the Si04 tetrahedra are part of three-membered rings. At a given alkali content, the abundance of these rings increases with increasing cation size. The abundance of three-membered rings in K-silicate melts correlates with a temperaUireand a non-linear composition dependence of the heat capacity. It is also a possible cause for the anomalous Volumetrie behaviorof potassium Silicate glasses. Because of the generally low sensitivity for 29Si NMR spectroscopy, the use of a Carr-Purcell- Meiboom-Gill (CPMG) echotrain to increase the sensitivity in glasses was investigated. The echo intensitydecay follows a stretched exponential behavior M(t) = M0 exp[-(t/t2)ß] with values for the exponent ß in the ränge of 0.41 to 0.65. The signal to noise in the spectra can be increased by a factor of up to 4 by taking the weighted sum of the spectra of the individual echoes. However, differential T2 relaxation for the different ff species is observed, with a shorter relaxation time for Q4 than Q. Thus, summing of the echoes leads to distorted spectral intensities and quantitative information can no longer be obtained from the spectra. To circumventthe problemwith differential T2 relaxation, an alternative approach is developed in which the spectral intensity for each chemical shift value is determined from the stretched exponential fit to its echo decay. With this approach,the sensitivity can be increased by a factor of up to 2.4, while quantitative informationcan still be obtained from the spectra. A 2ySi NMR spectrum with high signal to noise ratio was collected from a well characterized, isotopicallyenriched Si02 glass with a single scan after füll relaxation of the magnetization. Fromthe spectrum, the average Si-O-Si bond angle was estimated at 150° with a minimum fwhm of the bond angle distribution of 16°. The 29Si spin-lattice relaxation times were measured by Saturation recovery and differential Tl relaxation was observed for the various Si sites. In order to quantify the data from Raman spectroscopie measurements at high temperature, we developed a mathematical approach to interpret Raman spectra of binary Silicate glasses and melts withoutthe necessity of extemal calibration, e.g., from NMRspectroscopy, and without the need for any a priori assumptionsabout the linshape for the differentff species. We applied the quantification and the experimental approaches to investigate the speciation in potassium Silicate glasses. The equilibrium constant for the reaction 2Q3 = Q2 + Q4 was found to be logK3=-2.37±0.07,in excellent agreement withboth our own and previous NMRstudies. The enthalpy of formation(AH°) associated with the speciation reaction was determined to be 33.Ü7.3 kJ/mol with this approach. For the sodium Silicate system, we determined logK3 to be -1.88±0.04at room temperature, also in good agreement with NMRresults. The temperature dependence of the speciation is smaller for the sodium Silicate system than for the potassium Silicate system; AH° in sodium Silicate melts is 20.3±7.9 kJ/mol. Tn addition to the speciation, we also obtainedthe partial Raman spectra for the ff species for a ränge in temperatures for both Systems. By combining both isothennal and rate heating experiments, structural relaxation times were determined in the glass transition range with in situ, high-temperature Raman spectroscopy. The obtained relaxation times agree well with the shear relaxation times obtained from viscosity data. Thus, the timescale for viscous flow is the same as the timescale of the rearrangement of the Silicate network structure towards chemical equilibrium. This demonstrates that Si-O bond breaking is the primary control on Silicate melt viscosity.