On the Inherent Curvature of the Arrhenius Plot in Diffusion Experiments

The well known thermodynamic treatment of activated processes is used to determine the magnitude of inherent curvature of the Arrhenius plot, with specific application to the case of diffusion by means of a monovacancy mechanism. It is shown that the temperature dependence of the apparent activation energy Q is given by d Q /d T ≡ B = ΔC p where ΔC p is the difference in specific heat of the material containing an activated vacancy and of the perfect lattice. An estimate of B shows that | B |/ R ≦ 1.0 (where B is in molar units and R is the gas constant). On the other hand, available diffusion data for metals B ‐values which are an order of magnitude higher. It is therefore concluded that the observed curvature is not due to an inherent effect, but to the existence of more than one mechanism of diffusion.