A UNIVERSAL SEALING LAW FOR ATOMIC DIFFUSION IN CONDENSED MATTER

THERE is currently no unifying quantitative description of atomic diff usion in condensed matter, Analytic expressions have been obtained for the transport coefficients of an idealized dense fluid of hard sphere s(1,2), but their generalization to the rich variety of atomic structu res in real condensed systems remains a challenge, Here I present evid ence from molecular dynamics simulations that a universal relationship exists between the structure and the equilibrium rate of atomic diffu sion in liquids and solids. I find that the diffusion coefficient, red uced to a dimensionless form by scaling by the atomic collision freque ncy and the atomic diameter, is uniquely defined by the excess entropy , a measure of the number of accessible configurations of the system. A scaling law relating these two quantities holds well for simple liqu ids, and also remains applicable to atomic transport in a quasicrystal and to silver-ion diffusion in the solid-state ionic conductor alpha- AgI. This makes it possible to estimate diffusion coefficients directl y from diffraction measurements of an equilibrium structural character istic, namely the radial distribution function of the diffusing specie s.