Hello,
of course we can:
If you calculate the Diffusivity in x-direction, the entry in the main diagonal x_11 gives you the diffusivity in x-direction. The off-diagonal entries x_12, x_13, x_21 and x_31 give you the amount of diffusion going into y- and z-direction although the driving force, the concentration gradient, is only given in x -direction.
The same goes for Diffusivity in y- and z- direction.
Was that understandable?