Multicomponent diffusion in ionic crystals: theoretical model and application to combined tracer- and interdiffusion...

Area of Science:

• Materials Science
• Geochemistry
• Solid-State Physics

Background:

• Multicomponent diffusion in ionic crystals is complex, involving various mechanisms.
• Understanding these mechanisms is crucial for materials science and geochemistry.

Purpose of the Study:

• To develop a comprehensive model for multicomponent diffusion in ionic crystals.
• To apply this model to Na and K diffusion in alkali feldspar.

Main Methods:

• Developed a model incorporating vacancy-mediated diffusion and binary ionic exchange.
• Applied the model to alkali feldspar crystals using ${}^{41}$ K doped KCl diffusion couples.
• Annealed samples between 800-950 °C and analyzed concentration profiles using Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS).

Main Results:

• The Na self-diffusion coefficient was found to be at least 500 times higher than the K self-diffusion coefficient.
• Diffusion mediated by binary ${}^{39}$ K-${}^{41}$ K exchange was necessary for accurate model fits.
• The kinetic coefficient for K-K exchange was well-constrained.

Conclusions:

• The proposed model successfully describes multicomponent diffusion in ionic crystals.
• The findings highlight the significant difference in diffusion rates between Na and K in alkali feldspar.
• Binary exchange mechanisms play a critical role in the diffusion of multiple ionic species.