In its most general form, the Einstein equation relates the atomic mobility \(b_{i}\) of a species \(i\) to the diffusion coefficient \(D_{i}\): i.e., \(D_{i}=k\ T\ b_{i}\).
Source: PAC, 1999, 71, 1307. (Definitions of Terms for Diffusion in the Solid State) on page 1314 [Terms] [Paper]