The early days of high-resolution X-ray topography

Images of the surface or interior of a single crystal formed by X-rays Bragg-reflected from its lattice planes provide information about lattice misorientations and defects in a unique way that was appreciated by the pioneers of topographic techniques: Berg, Barrett, Guinier, Ramachandran and Wooster. High-resolution images were achieved when use of fine-grain photographic emulsions was combined with diffraction geometries, providing micrometre-scale geometrical resolution. The detection of individual dislocations by X-ray diffraction contrast was reported in 1958 by three laboratories independently, employing quite different diffraction geometries. The value of X-ray diffraction contrast as a method of general application in detecting and identifying lattice defects (as opposed to special methods such as chemical etching) was demonstrated principally by the geometrically simply interpretable images of the projection topograph. Within a few months in 1958 this technique showed how dislocation Burgers vectors could be determined, how stereo-pairs of images could be formed, and how structure amplitudes could be measured absolutely from Pendellosung fringes in images of wedge-shaped crystals (though it was the hook-shaped Pendellosung fringe patterns appearing in section topographs that led Kato to develop his spherical-wave diffraction theory). The 'failure of Friedel's Law' revealed on early stereo-pairs of dislocation images suggested that sense as well as direction of Burgers vectors could be determined: confirmation came from the experiments of Hart applying the 'refraction of energy-flow' theory of Penning and Polder. X-ray topographic imaging by Polcarova and Lang of internal magnetic domain structures on the scale of a few micrometres in Fe-Si (magnetostriction constant 2.7*10 -5 ) confirmed the high strain-sensitivity of the method and its ability to explore phenomena not accessible by other investigative techniques.