184.108.40.206 Diffuse scattering of X-rays or neutrons, Huang scattering (DXS)
This chapter discusses differential dilatometry (DD), diffuse scattering of X-rays or neutrons and Huang scattering (DXS). The change of the lattice parameter Δa/a is directly related to the volume relaxation Vrel of the defects or to the trace of the dipole tensor Tr P that describes their displacement field in the elastic continuum. In general, however, the comparison of Δa/a and Δl/l (DD) yields directly the number of lattice sites created or eliminated by the defects. In this manner the method has been successfully used to determine the defect concentration in high-temperature equilibrium. Due to the limited experimental accuracy the method is limited to rather high defect concentrations: i.e. temperatures close to the melting temperature. However, as the method determines the concentration of vacancies independent of their structure, the contributions of divacancies cannot be separated. Assuming a low concentration and statistical distribution of the defects, the diffuse scattering cross section is given by the product of the defect concentration c, and the square of the scattering amplitude due to one defect. From measurements over larger regions in reciprocal space detailed information on the defects structure and their displacement fields can be obtained. If only the comparatively high intensity close to the Bragg reflection is available, separate results on the defect concentration and the dipole tensor can be obtained by combination with measurements of Δa/a. For a detailed investigation of point defects, defect concentrations of > 100 ppm are necessary with the present experimental techniques.