The application of 17O and 27Al solid-state (3QMAS) NMR to structures of non-crystalline silicates at high pressure

Abstract

The recent development in two-dimensional solid-state triple quantum (3Q) magic angle spinning (MAS) NMR offers much improved resolution compared with conventional one-dimensional MAS NMR, allowing the structural details of amorphous silicates to be revealed. Since the first experimental application of 3QMAS NMR to the silicate glasses quenched from melts at high pressure in a multi-anvil apparatus [Lee, S. K., Fei, Y., Cody, G. D., Mysen, B. O., 2003a. Geophys. Res. Lett. 30, 1845], there has been continued progress in the understanding of the structures of silicate melts at high pressure. Here, we present the recent progress and insights made by O and 27 Al 3QMAS NMR spectra of silicate glasses quenched from melts at pressures up to 10 GPa in a multi-anvil apparatus, revealing new details of melt structures at high pressure . The atomic structure of sodium silicate and aluminosilicate glasses at high pressure is significantly different from that at ambient pressure. There is evidence of extensive cation ordering among highly coordinated network polyhedra, such as [5,6] Al and [5,6] Si, that affect corresponding thermodynamic and transport properties. New oxygen sites are observed at high pressure in a series of silicate glasses with varying degree of polymerization, e.g. [5,6] Al–O– [4] Si, [5,6] Si–O– [4] Si, and Na–O– [5,6] Si. The fractions of these clusters tend to increase with pressure with corresponding reduction of non-bridging oxygen (Na–O– [4] Si), thus increasing the net degree of polymerization. The fraction of [5,6] Al in aluminosilicate glasses increases with pressure, consistent with previous studies, but decreases with increasing degree of polymerization of melts from depolymerized to fully polymerized melts at an isobaric condition. The bond angle and length distribution, as well as the range in distortion of framework units increase with increasing pressure, which increases the topological entropy. These results shed light on a new opportunity for studying structures of silicate glasses and melts at high pressure and help to provide microscopic constraints for the melt properties in the Earth's interior .