diffusion under the influence of a gradient in chemical composition. In concentrated solid solutions, e.g.
A1-xBx, or in diffusion couples, the motion of one constituent causes a counterflow of the other constituents(s) or vacancies. In this case one can define a diffusion coefficient for the intermixing, which is called the chemical diffusion coefficient or interdiffusion coefficient.
Source:
PAC, 1999, 71, 1307. 'Definitions of Terms for Diffusion in the Solid State' on page 1310 (https://doi.org/10.1351/pac199971071307)