The Correlation Between Diffusion Behaviour and Phonon Softening in BCC Metals

A new approach is proposed to explain the wide spread of self diffusion behaviour and the curvature of the Arrhenius plots in bcc metals by typical diffusion-relevant low-energy phonon modes. In bcc transition metals the activation enthalpy and the energy of the LA 2/3 (111) phonon mode decrease systematically with decreasing d-electron concentration, while the diffusivity and the degree of curvature increase. The migration enthalpy HM is related to the frequency of this mode, i.e. to the amplitude of atomic displacements in the 〈111〉 nearest neighbour jump direction. On the basis of experimental results a good correlation between the square of this phonon frequency and the activation enthalpy is obtained. The curvatures of the Arrhenius plots are explained by a decrease of HM with decreasing temperature due to the temperature dependence of the soft T1A 1/2 〈110〉 mode, which causes a “breathing” motion of the saddle point atoms. Thus the model of phonon-assisted diffusion jumps via monovacancies yields a uniform explanation for the main features of diffusion in bcc metals.