Thermal diffuse scattering of X-rays and neutrons

Abstract

Thermal motion of the atoms in a crystal leads to a reduction in the intensities of the Bragg reflections and to a diffuse distribution of non-Bragg scattering (known as thermal diffuse scattering) in the rest of reciprocal space. Thermal diffuse scattering gives information about the lattice dynamics of the crystal, i.e. about the amplitudes and frequencies of the small oscillatory displacements of the atoms from their equilibrium positions which arises from thermal motion. This chapter gives a brief account of the classical Born–von Kármán treatment of lattice dynamics underlying the interpretation of experimental data on thermal diffuse scattering. This is followed by the theory of the scattering of X-rays and neutrons by thermal vibrations in a single crystal. The thermal diffuse scattering of X-rays or neutrons is used principally for the measurement of frequency versus wavevector (phonon dispersion relations). Although the first measurements were obtained with X-rays, these experiments are carried out more readily with neutrons where the particle energies are comparable with phonon energies. A summary is given of the derivation of interatomic force constants and of elastic constants from the dispersion relations. This chapter is also available as HTML from the International Tables Online site hosted by the IUCr.