Pressure-Induced Structural Transitions in Na-Li Silicate Glasses under Compression

Abstract

The atomic structures of alkali silicate glasses with more than two types of nonframework cations (e.g., Li and Na) at high pressure provide insights into the effects of pressure on the transport properties of technologically important complex oxide glasses and multicomponent silicate melts in the Earth interiors. Despite the importance, the pressure-induced changes in the atomic structures of Na-Li silicate glasses at high pressure have not been studied. Here, we investigated the effects of pressure on the coordination environments around Si and O and the degree of mixing among alkali cations in amorphous Li-Na silicates quenched from melts at high pressure up to 8 GPa using Li-7, O-17, and Si-29 NMR spectroscopy: The proportions of Si-[5,Si-6] increase with increasing pressure and tend to decrease with increasing X-Li [=Li/(Li + Na)], confirming that an increase in X-Li could prohibit the formation of Si-[5,Si-6] species in mixed-alkali silicate glasses. Li-7 magic angle spinning NMR results reveal a pressure-induced increase in the Li coordination number and shortening in Li -Li distance stemming from reduction in free volume and the changes in the degree of cation mixing. The O-17 NMR spectra confirmed that the fraction of nonbridging oxygen (NBO) [i.e., (Na,Li)-O-Si-[4]] decreases as pressure increases. The decrease in NBO is more prevalent in Li-poor glass, indicating that the degree of network polymerization at high pressure depends on the Li-Na ratio. While the threshold of the difference between ionic radii in chemical order has been suggested to be similar to 0.3 angstrom at 1 atm, chemical order among network modifiers may emerge with ionic radii difference smaller than 0.3 angstrom at high pressure. Our NMR results indicate that decreasing numbers of similar pairs as pressure increases tend to decrease the diffusivity of Li within Na-Li trisilicate glasses.