Chapter 12 - The application of 17O and 27Al solid-state (3QMAS) NMR to structures of non-crystalline silicates at high pressure

Abstract

This chapter presents the recent progress and insights made by 17O and 27Al two-dimensional solid-state triple quantum (3Q) magic angle spinning (MAS). The chapter summarizes the way to obtain the quantification of disorder in silicate glasses and melts from the solid-state NMR techniques. The results help to establish the classification scheme of disorder in non-crystalline silicates. Nuclear magnetic resonance (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 recent development in 3Q MAS NMR offers much improved resolution as 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, there has been continued progress in the understanding of the structures of silicate melts at high pressure. These experimental results highlight a new opportunity to investigate the molecular structures of silicate melts at high pressure and reveal connections between the microscopic signatures of pressure-induced changes with the macroscopic properties of melts. The chapter also explores the pressure-induced changes in the coordination environment of framework units and the corresponding connectivity reflected in bridging oxygen (BO) and non-bridging oxygen (NBO) environment at the high pressure. Details of connectivity in silicate glasses and melts at high pressure include a decrease in the fraction of NBO with increasing pressure because of multiple densification mechanisms at distinct pressure ranges, and the formation of new oxygen clusters, linking these highly coordinated framework units.