Self-diffusion and inter-diffusion coefficients of liquid lithium isotopes

Experimental data on Li-6 and Li-7 diffusivities, as determined by Feinauer et al. [A. Feinauer, G. Majer and A. Seeger, J. Phys. Condensed Matter 6 L355 (1994)], are discussed in the framework of a theory previously developed by Omini [M. Omini, Phil. Mag. A 54 561 (1986)]. The scattering processes responsible for diffusion are described in terms of a Fourier-transformable pair potential through which the diffusing atom interacts with the atoms of the surrounding liquid. Two main improvements are introduced into the theory with respect to its original formulation: the first is represented by a more reliable form of trial function in the variational procedure used to solve the transport equation; the second consists of a reinterpretation of the long wavelength part of the potential, i.e. the part connected with collective modes of vibration. The theory provides an explanation of the dynamic isotope effects observed in self-diffusion and inter-diffusion coefficients.