Sung Keun Lee, George D. Cody, Yingwei Fei, Bjorn Mysen
Geophysical Laboratory, Carnegie Institution of Washington
s.lee@gl.ciw.edu
The extent of disorder and thus the structures of amorphous solids at high-pressure provides important constraints on the transport and thermodynamic properties of silicate magmas at high pressure. Whereas some data on pressure-induced coordination changes of framework cations in oxide glasses and melts exist and the strong link between the extent of intermixing among network units and macroscopic properties were recently established for melts at ambient pressure (e.g. Lee and Stebbins J. Phys. Chem. B. 2003, p3141, Lee, Cody, Mysen, Phys. Rev. B. accepted), the extent of intermixing and disorder among these highly coordinated network polyhedra (e.g. [6]Si, [5]Si, [5,6]Al) at high pressure are far from being understood. Here we present first experimental evidence supporting chemical ordering among network polyhedra in sodium silicate and sodium aluminosilicate glasses quenched from melts at 6-10 GPa by using high-resolution NMR, which resolves new oxygen sites at high pressure, including [5,6]Al-O-[4]Si, [5,6]Si-O-[4]Si and Na-O-[5,6]Si. Improved resolution of 2-dimensional O-17 MQMAS (multiple quantum magic angle spinning) NMR spectra enables us to evaluate the extent of intermixing among network polyhedra at high pressure, whose distribution is deviated from random distribution but favors formation of bridging oxygens linking dissimilar pairs such as [5,6]Si-O-[4]Si or [5,6]Al-O-[4]Si. Quantum chemical calculations were used to assign these new oxygen clusters for the NMR spectra. For instance, the calculated isotropic chemical shift of [n]Si (or Al) -O-[4]Si is more deshielded with increasing n from 4 to 6. We explored the atomistic origins of ionic diffusivity and related thermodynamic properties of melts at high pressure using statistical mechanical modeling and NMR results. The methods and results given here provide improved prospects for atomistic origin of magmatic processes in the Earth's interior as well as fundamental problems of the structures of melts at high pressure.