An Ionic Boundary Layer near the Lithium Niobate Surface in the Proton Exchange Process

The paper presents a theoretical study of the behavior of an ionic boundary layer that occurs in the process of a steady proton exchange in a benzoic acid melt contacting with the surface of a lithium niobate crystal. The penetration of protons into a crystal promotes the injection of oppositely charged ions (lithium and benzoate) from the surface of lithium niobate in the surrounding acid. The transfer of the reaction products and their interaction in benzoic acid is studied numerically. The proposed mathematical model includes the effect of recombination in the volume so that the ions with different charge signs approach each other and form a neutral lithium benzoate. The results of the numerical simulations demonstrate that there are exponential-like concentration profiles of two types of ions, and a non-uniform electric field and pressure distributions develop in the boundary layer under steady-state conditions. In this process, the total charge of the system remains zero. It is shown how the concentration, recombination, and activity of lithium and benzoate ions and the diffusion coefficients affect the profile shapes and the process intensity.