Measuring Self-Diffusion Coefficients up to 1500 K: A Powerful Tool to Investigate the Dynamics and the Local Structure of Inorganic Melts

Self-diffusion is a fundamental property of liquid dynamics that also provides important structural information. To explore the dynamics in inorganic melts with high liquidus temperature, we propose a new setup based on pulsed field gradient NMR combined with laser heating that makes possible in situ self-diffusion coefficient measurements up to 1500 K. Applied to several corrosive molten fluorides in a wide range of compositions and temperature, we have evidenced the different key parameters of their motion along with their structural characteristics. In alkali fluorides, the self-diffusion coefficient of fluorine depends slightly on the composition compared to the temperature, displaying these systems as an ideal bath of polarizable hard spheres. In contrast, self-diffusion in rare earth and alkali fluorides mixtures presents a complicated balance between temperature and the network-forming process of the ionic long-lived units. These results open wide perspectives in the study of high temperature liquids.