The variation of quasicrystal defect contrast: how and why

We carried out a systematic study of typical defect contrasts in various si ngle grains of icosahedral Al-Pd-Mn quasicrystals. White-beam topographs wi th different sample-to-detector distances and multiple-crystal topographs w ith different sample-to-detector distances, different working points on the rocking curve and different harmonic reflections have been recorded. Despi te the extreme geometrical resolution at the ID19 topography beamline of th e ESRF, white-beam topography showed reasonable (not blurred) contrast only for very short sample-to-detector distances (<4 cm). Depending on the defe ct type, in synchrotron multiple-crystal topography the defect contrast cha nges considerably as a function of the position of the working point on the rocking curve. Under the special conditions chosen, the dependence of cont rast shape and size on the harmonic reflection used was rather weak.