Intrinsic proton dynamics in hydrous silicate melts as seen by quasielastic neutron scattering at elevated temperature and pressure

We present quasielastic neutron scattering results on hydrous silica, sodium aluminosilicate, and sodium trisilicate melts with 10 mol% total water content, studied at high temperature under high pressure. Combining neutron time-of-flight spectrometry with neutron backscattering, intrinsic, microscopic proton dynamics is investigated on a time scale from 0.2 ps up to 1 ns between 850 K and 1250 K. All three hydrous silicate melts exhibit a relatively slow proton dynamics, although the melt viscosity is drastically reduced upon water dissolution. The self-diffusion coefficient of proton in the hydrous sodium trisilicate melt is on the order of 10 ⁻¹¹ m² s ⁻¹, two orders of magnitude slower than the sodium dynamics in the corresponding anhydrous melt. The proton dynamics in hydrous silica and albite is not faster than that time scale. We show that the transport mechanism involves not only –OH but also molecular water species. All protons are mobile during the transport of the water instead of diffusion of a specific water speciation. These characteristics of the proton structural relaxation in the melt can be attributed to a transport in a complex H-bonding environment.