Yves Epelboin

X-ray dynamical diffraction: the concept of a locally plane wave

The long distance between the source and the experiment and the small source size, now available at third-generation synchrotron sources, leads to new optical characteristics for X-ray diffraction. It is shown that, under certain conditions, the intensity received by a point located on the exit surface of a crystal is described by the diffraction of a locally plane wave. Each point along the surface is influenced by a plane wave with a varying departure from Bragg angle. In such a case, it is possible to visualize the rocking curve of the crystal as a function of the position along the exit surface. This represents a topographic method to obtain the reflectivity curve of the crystal instead of the usual goniometric method. Applications will be described in a forthcoming paper.

Characterization of dislocations in protein crystals by means of synchrotron double-crystal topography

Hen egg-white lysozyme (HEWL) crystals have been studied by means of double-crystal synchrotron topography. The crystals reveal a number of features that are quite well known in hydrothermally grown inorganic crystals: dislocations, growth bands and growth sector boundaries. Dislocations in the (110) sectors have been characterized as edge dislocations with Burgers vector parallel to the c axis. They are distinguishable only under weak beam conditions. The presence of edge dislocations shown in this paper is consistent with the spiral growth steps previously reported. This spiral growth on protein crystals has been observed many times by surface techniques.

New contrasts of piezoelectric devices using synchrotron stroboscopic section topography at ESRF

The propagation modes of ultra-acoustic waves in quartz planar resonators have been studied by means of stroboscopic section topographs at ESRF. The images present a number of internal lines inside the section that allow a quantitative analysis of the vibration modes. When the excitation level increases, the shape of the lines enables the study of the dumping of the vibration along the surface of the device. Such analysis was made possible because of the small divergence of the white beam delivered by a third-generation synchrotron source.

Three-dimensional computer reconstructions from serial sections of cell nuclei

The analysis of ultrathin serial sections as 3‐dimensional (3D) information requires interpretation and display of a large amount of data. We suggest a simple way to solve this problem; it permits presentation of a series of sections as a 3D color image of good quality. It involves a picture system with specialized hardware and software written for this purpose.

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.

New Materials and new Devices for Filtering in Radio-communication Systems

There is now a large market for the UHF filters made with piezoelectric resonators due to their large use in radio-communication systems. These filters are presently built with materials presenting only a moderate coupling coefficient and using very similar circuit topologies. In this contribution we demonstrate that this lead to a limitation of their performances and we discuss solutions to improve them. These solutions are found considering alternative filter topologies and/or other materials and/or new kinds of devices (resonators or monolithic filters)

The Use of an Array Processor for the Simulation of X-ray Topographs

The use of a varying-step algorithm now allows the simulation of traverse topographs and of section topographs taking into account the real width of the incident beam. However, computation time remains a critical factor in practical use. With an array processor it is possible to decrease the computation time significantly. It is shown that pictures of good quality may be obtained in a reasonable time using local facilities. The influence of various parameters on the accuracy of the simulations is discussed. It is demonstrated that local machines can be more useful, in crystallography, than giant computers often difficult to reach through the network of communications.

White-beam topography of Rayleigh waves: a numerical study

This paper analyses the contrast of white-beam stroboscopic synchrotron topographs of Rayleigh waves induced by a piezoelectric transducer in an α-quartz crystal. The aim of the present investigation is to study the influence of the crystal thickness and of the vibration amplitude on the image contrast. A periodic optimal thickness is found that maximizes the film contrast.

Simulation of X‐ray traverse topographs by means of a computer

The precision of the numerical algorithm used is the critical factor in computing traverse topographs. The varying-step algorithm allows such a computation. Images have been computed simulating the real experiment, i.e. by the addition of the intensities of individual section topographs. Results are in good agreement with experiments: it has been possible to characterize dislocations fully by the study of the fine details of their contrast. This may be of practical interest whenever section topographs cannot be used, as, for instance, in the case of thin crystals.

Crystalline quality of the trigonal piezoelectric materials and effects of the extended defects

Many frequently used or promising piezoelectric materials belong to crystal classes 32 or 3m. Among them are α quartz and its crystallographic analogs (AlPO4, GaPO4, α-GeO2, etc.), the numerous materials of the langasite (La3Ga5SiO14) family and also lithium tantalate (LiTaO3) and lithium niobate (LiNbO3). In this paper we study the present state of the art for these materials, indicate their principal point and extended defects, and present methods to reduce the dislocation density. Large concentrations of intrinsic point defects often exist in crystal grown at very high temperatures. The point defects (intrinsic or related to impurities) modify the constants and can increase the acoustic losses. This is the case for the alkali ions and the OH that induce severe losses in different temperature intervals. The extended defects also affect the performances of the piezoelectric devices. Some, such as twins, ferroelectric domains, or large solid or liquid inclusions, have very detrimental effects. Dislocations, growth bands, and planar defects are more difficult to avoid and affect the devices in a more subtle manner. In quartz and its analogs, dislocations seem to increase the nonlinear elastic effects and have a collective effect on the vibration modes, particularly in energy trapping resonators. Growth bands and stacking faults also produce similar effects.