Martin Wilding completed his first degree at Derbyshire College of Higher Education (now the University of Derby) and completed a PhD at the University of Edinburgh in 1990. He has considerable research experience and uses advanced diffraction techniques and novel sample environments to study materials under extreme conditions. His interests include the structure and glass and liquid structure including novel glass forming liquids and molten salts. He is currently Senior Experimental Officer at the University of Manchester at Harwell.
Martin C. Wilding1,2,3, Brian L. Phillips3, Mark Wilson4, Geetu Sharma5, Alexandra Navrotsky5, Paul A. Bingham1, Richard Brooker6, John B. Parise3
1Materials and Engineering Research Institute, Sheffield Hallam University, Howard Street Sheffield S1 WB UK 2University of Manchester at Harwell, Diamond Light Source, Harwell campus, Didcot, OX11 0DE UK 3Department of Geosciences and Department of Chemistry, Stony Brook University, Stony Brook, NY 11794-2100, USA 4Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ UK 5Peter A. Rock Thermochemistry Laboratory, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA. 6School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, U.K
Although carbonates rarely form glasses, some melt compositions in the middle of the K2CO3-MgCO3 join can be quenched. Despite being first reported in 1929 the formation of glasses in this system is poorly understood. Unlike more conventional and commercially important silicate, aluminosilicate and borosilicate systems, carbonate, and other ionic glass-forming systems such as nitrates and sulphates, lack conventional network-forming components and in this study we have used several different experimental techniques to determine the formation mechanism of this exotic carbonate glass. We have made a 0.55K2CO3-0.45MgCO3 composition glass using high pressure synthesis techniques and have used drop solution calorimetry measurements using molten sodium molybdate (3Na2O. MoO3) at 975 K to determine the enthalpy of formation. The enthalpy of formation is 115.00 + 1.21 kJ/mol at 298K and the corresponding heat of formation from oxides at 298 K is -261.12 + 3.02 kJ/mol. These values show that the glass is metastable in enthalpy relative to the crystalline end-members and has a small, effective heat of vitrification of +9.36 + 1.95 kJ/mol. This suggests that the glass is stabilised by an entropic term.
Attenuated Total Reflectance (ATR) infrared spectroscopy show two distinct v1 symmetric stretching and two v2 out-of-plane bending peaks. This strongly suggests there are two distinct carbonate groups present in contrast to crystalline carbonates and further supports a high degree of disorder. In addition 13C MAS NMR spectroscopy has been performed on isotopically enriched glass, this shows a broad, nearly symmetrical center band at 168.7 ppm, with a FWHM of 4.7 ppm typical for alkali and alkaline earth carbonates. The large peak width indicates the presence of a significant degree of disorder and corresponds to a broad distribution of chemical shifts in the glass that spans the chemical shift range for carbonate groups. More information on the distortion of the carbonate anions is extracted by determining the anisotropy and asymmetry of the chemical shift tensor. By using slower rotation MAS techniques the principal axis values of the chemical shift tensor can be determined from the complex spinning sideband pattern. The anisotropy is similar to that observed for amorphous calcium carbonate and suggest only limited departure of the carbonate group from planar geometry. The value for the asymmetry determined however is significantly larger than any previously reported value. Both sets of spectroscopy data and the calorimetry measurements support a simulation approach that uses a flexible anion approach.
Molecular dynamics simulations in which the molecular anion is allowed to be flexible have been used successfully to model the diffraction patterns from carbonate liquids. We have used the same approach to simulate the K2CO3-MgCO3 liquids and although there is no evidence that carbonates form chains or the other complex structures there clear evidence that an additional C-O length scale is beginning to develop and the ambient pressure structure shows isolated, distorted carbonate anions with different C-O bond lengths consistent with the in-plane distortion identified by the NMR. The interaction between the potassium cations and the oxygens from the distorted carbonate anions defines the glass structure at low pressure and is also reflected in the response of these carbonates to high pressure.