Some peculiarities of Ti in-diffusion in lithium niobate

The process of one- and two-dimensional Ti in-diffusion in lithium niobate, LiNbO₃ (LN), single crystals of <x> and <z> orientations at 950-1060°C in oxygen/water vapor medium had been studied. In the case of one-dimensional diffusion, a flat diffusion front consisting of Ti solid solution in LN is formed. The process kinetics is described by Fick's equation for the case of a permanent source. In the case of two-dimensional Ti diffusion, the diffusion zone contour acquires the shape close to elliptical. The diffusion rate in the tangential direction is about an order of magnitude higher than in the normal direction. A model qualitatively describing such nontrivial character of the diffusion process is suggested. The model is based on: a) incongruent lithium evaporation out of LN at high temperatures; b) low thermodynamic activity of Li in LiTiO₃ - product of Ti interaction with LN. These factors produce a significant nonlinearity of the process, since a strong chemical bond between Li and Ti in this phase leads to a mutual increase in their diffusive mobility.