Neutron scattering studies of diffusion in solids

This chapter shows how the scattering of slow neutrons can be used to learn about the microscopic mechanisms of atomic diffusion in solids. It discusses properties of the neutron; scattering experiments and spectrometers; types of diffusion studies; the neutron scattering cross-section; coherent and incoherent scattering; scattering functions and Van Hove correlation functions; interpretation of correlation functions in the classical approximation; long-time limit and elastic scattering; self-correlation and incoherent scattering functions for specific models; separation of different motions; continuous translational and rotational diffusion; diffusion by discrete jumps; explicit calculation of the incoherent scattering function for local atomic jump diffusion between two sites; three examples of local jump diffusion; diffusion on a lattice; fast ion diffusion in solid electrolytes; and phenomenological treatment of coherent quasielastic scattering. Advantages of the neutron method are that it does not require a macroscopic concentration gradient (the diffusion process can therefore be studied in homogeneous systems at equilibrium and with well-defined concentrations of mobile atoms); it is non-destructive (neutron measurements of diffusion can be carried out on a single sample); and neutron experiments can be performed under extreme conditions of temperature and pressure