André Authier

Seitz symbols for crystallographic symmetry operations

The aim of this report is to describe the Seitz notation for symmetry operations adopted by the Commission on Crystallographic Nomenclature as the standard convention for Seitz symbolism of the International Union of Crystallography. The established notation follows the existing crystallographic conventions in the descriptions of symmetry operations.

60 years of IUCr journals

In the 60 years since its birth in 1948, the number of journals published by the International Union of Crystallography has risen from one to eight. A brief account of the history of the forerunner of the IUCr journals, Zeitschrift für Kristallographie, is given. The context of the birth of the IUCr and the first of its journals, Acta Crystallographica, is recalled. The circumstances which led to the growth of Acta into several sections, at first A and B then, successively, C, D, E and F, and the launch of two new journals, the Journal of Applied Crystallography and the Journal of Synchrotron Radiation, are described. The transition from print-on-paper to electronic journals is also remembered.

Experimental and computer simulation study of the variation with depth of the X-ray section topograph images of a dislocation

A systematic study of the variations of the contrast of a dislocation in silicon on section topographs with the depth of the line was performed both experimentally and with computer simulations. Mo Kα1 radiation and 33{\bar 3} and {\bar 3}{\bar 3}3 symmetric reflections were used. The crystal thickness was 440 μm so that the value of μt was 0.64. The influence of the orientation of the dislocation was studied for values of the angle between the line and its Burgers vector ranging between 60 and 90° in the glide plane. It was observed that when the dislocation lies close to the entrance surface, whatever its orientation, its image is centred around the trace of the plane of incidence passing through the intersection of the dislocation with the direct beam while when the dislocation lies close to the exit surface its image is centred around the projection of the dislocation on the section pattern. The variation of the orientation of the image for intermediate depths of the dislocation is interpreted by means of the geometrical construction of the dynamical image. The values of the orientation of the image calculated according to this simple model are in good agreement with those measured on both experimental and simulated topographs. The same geometrical model enables the difference in the relative positions of the direct and dynamic images of stereo pairs to be explained. A new feature was observed in the simulated images and several of the experimental ones, namely a concentration of intensity along the projection of the dislocation in the reflected direction. Slit width was taken into account in the simulations for a better fit with experimental topographs but not polarization, which was taken to be normal to the plane of incidence. Because of the small value of the crystal thickness and of μt, the variation of the contrast with the Burgers vector is very small, making its determination very difficult.

Optical properties of X-rays – dynamical diffraction

The first attempts at measuring the optical properties of X-rays such as refraction, reflection and diffraction are described. The main ideas forming the basis of Ewalds thesis in 1912 are then summarized. The first extension of Ewalds thesis to the X-ray case is the introduction of the reciprocal lattice. In the next step, the principles of the three versions of the dynamical theory of diffraction, by Darwin, Ewald and Laue, are given. It is shown how the comparison of the dynamical and geometrical theories of diffraction led Darwin to propose his extinction theory. The main optical properties of X-ray wavefields at the Bragg incidence are then reviewed: Pendellösung, shift of the Bragg peak, fine structure of Kossel lines, standing waves, anomalous absorption, paths of wavefields inside the crystal, Borrmann fan and double refraction. Lastly, some of the modern applications of the dynamical theory are briefly outlined: X-ray topography, location of adsorbed atoms at crystal surfaces, optical devices for synchrotron radiation and X-ray interferometry.

The birth of the European Crystallographic Committee (ECC) and of the European Crystallographic Meetings (ECMs)

At ECM25, held in Istanbul, Turkey, on 16–21 August 2009, the 25th anniversary of the European Crystallographic Meetings was celebrated. In this article, it is recalled how the idea of coordinating international meetings on crystallography in Europe was put forward at a meeting held in Manchester, UK, in April 1971, and it is explained how the European Crystallographic Committee was established for that purpose during the Ninth IUCr Congress in Kyoto, in 1972. The organization of the first European Crystallographic Meeting, ECM1, held in Bordeaux, France, in 1973, is briefly described and the evolution of the main topics of the scientific programme from that time to ECM25 is commented upon.

A note on Bragg-case Pendellösung and dispersion surface

It is usual to state, since Ewald [Ann. Phys. (1917), 54, 519-597], that Bragg-case Pendellösung involves two wavefields belonging to the same branch of the dispersion surface. However, in the Bragg geometry or when crystals are highly absorbing, the dispersion surface can no longer be approximated by the dispersion surface valid for a non-absorbing infinite medium. When considering the real part of the dispersion surface, it is found that the interfering wavefields producing Pendellösung in the Bragg geometry belong in fact to different branches. This paradox is explained by noting that the actual dispersion surface in the semi-infinite medium where the wavefields are generated by the incident wave differs from the dispersion surface in an infinite medium.