Diffusion studies of solids by quasielastic neutron scattering

Abstract

The diffusion in solids is usually investigated on a macroscopic scale, for instance following the interpenetration of two atomic species across an interface which is related to the chemical diffusion coefficient. On the other hand, the self-diffusion coefficient is measured by labelling atoms with radioactive isotopes. These techniques have been presented in Chap. 1. Labelling can also be achieved by Larmor precession in a magnetic field gradient where the corresponding dephasing depends on the diffusive migration of the precessing particles (Chaps. 10 and 17). In contrast to such "macroscopic" methods the diffusion process can be studied for atomistic time scales by the spinlattice relaxation in nuclear magnetic resonance spectroscopy (Chap. 9). The method of quasielastic neutron scattering (QENS)1, which focuses on scattering processes at small energy transfers, penetrates into the region of 10 -13 to 10-8 s. In addition, quasielastic neutron scattering explores the diffusive motion in space, for a range comparable with the wavelengths of the neutrons used in spectroscopy. Typical correlation lengths, diffusive paths, jump distances, and vibrational displacements can be evaluated between 10-6 and 10-9 cm. Inelastic neutron scattering on such diffusive processes leads to a spectrum of energy transfers.