Diffraction topography using white X-ray beams with low effective divergence

The divergence of the incident X-ray beam as seen from a point in the specimen, the effective divergence α, is of the order of a microradian at a third generation synchrotron radiation source. This entails two effects on white- beam X-ray diffraction topography. 1. The specimen-detector distance can be varied at will in the metre range without appreciable blurring of the image. Thus the discontinuous change in distortion associated with magnetic domains, or implanted layers in a piezoelectric material, can often be directly measured. It was also possible to observe focusing effect due to continuous spatial variations of lattice plane orientation. This effect was visualized in the cases of the images of single dislocations, from elastic resonance patterns, and on wafer-bonded samples. 2. The small value of α can also be described as yielding appreciable lateral coherence of the beam. Propagation of the diffracted beam, i.e. Fresnel diffraction, can turn variations of the phase of the diffracted beam into changes in intensity, hence contrast. In the case of a periodic spatial variation of the phase due to periodic poling, the Talbot effect in the diffracted beam provides the possibility of measuring the difference in phase of a structure factor in inversion-related domains, i.e. for Friedel pairs.