IUCr 1994 Report - Commission on Crystallographic Apparatus

During 1994, the Commission continued its activities in the field of lattice-parameter determination, absolute structure determination for light atoms, two-dimensional radiation detectors and high-pressure crystallography. A brief report of the projects being undertaken is given below.

(1) Precision and Accuracy of Lattice Parameter Determination (G. DeTitta and L. Finger). The project aims at the determination of realistic limits on the precision and accuracy of lattice parameters as reported in the structural crystallography literature. It has in its scope to determine the strengths and weaknesses of various approaches, assess the important issues in an accurate and precise measurement, devise tools to aid in the measurement and to characterize the equipment used in these determinations. Two main thrusts of the project are (1) a round-robin experiment in which structural crystallographers are asked to determine the lattice parameters for an unknown sample and a known standard sample, and (2) to develop a standard reference material and associated instructional material to allow the structural crystallographer both to gauge the quality of lattice-parameter determination coming out of the laboratories and to improve that quality.

With support from the IUCr and NIST, kits containing properly characterized ruby crystals and five zeolite crystals, mounted and ready for lattice-parameter determination, have been prepared and nine kits have been shipped to participants in the round-robin project. Kits are in South Korea, Russia, the US, Canada, the UK and France. As yet we have not received any results but we expect them soon. With the money that NIST made available, 4000 ruby spheres were purchased; NIST plans to turn them into a SRM (standard reference material). The IUCr fund made it possible to hire a masters student for the summer to handle the details of the packaging and shipment of the kits.

The zeolite crystals are thin plates that have been mounted in such a manner that the plate normals are not exactly perpendicular to the phi axis, making their optical alignment non-trivial. On the other hand, alignment of the rubies is straightforward. What the results are expected to show is that problems with lattice-parameter determination have more to do with misalignment of the crystal and/or of the diffractometer than any other fundamental problem. Results with the ruby spheres should give us information about the alignment of the diffractometers, which we will be able to include in our analysis of the zeolite results. Since the actual cell constants for both samples are well known, we can interpret the reported cell constants as perturbations of the true constants owing to beam nonhomogeneities (spatial and spectral), misalignments of the crystal/diffractometer, misidentification of the wavelength operative for each reflection centered, centering methods etc. Participants are being asked to report the actual diffractometer angles used in the determination of the cell parameters.

(2) The Absolute Structure Determination of Light Atom Compounds (ASDLA) (L. Malakhova and E. F. Weckert). This project aims at a comparison of various methods used for the determination of the absolute structure of light-atom compounds and a generalization of the experimental and practical experience accumulated by scientific laboratories. Since the first determination by Bijvoet (1951), absolute structures have been receiving increasing attention, in particular in the field of biological compounds and pharmacology. The anomalous-scattering technique is well established now but the technique is not applicable to biological and organic crystals including only light atoms (H, C, N, O) and no heavy atoms. This problem has led to the use of complicated apparatus (synchrotron radiation) or to long-lasting and careful experiments or to the appearance of novel experimental techniques for the determination of the absolute structure (direct measurements of triplet phases, use of three-beam interaction). The present project involves the determination of the absolute structure of a given series of crystals with different characteristics by all project participants with the use of techniques and apparatus available in respective laboratories. The data obtained should allow us to establish the numerical correlation between the sample characteristics, parameters of the experimental apparatus used and the results of the absolute-structure determination by different techniques.

According to the proposed time schedule encompassing 1994 to 1996, the key points of the ASDLA project, suggested for consideration for the project participants, were discussed, added and approved in essence in 1994. The crystal substances suitable for the project performance were chosen after discussion among the participants. Information about the variety of the absolute-structure-determination techniques as well as apparatuses available to the project participants shows good conditions to carry out the project. We are about to do the first experiments. Out of 11 laboratories originally planned, 7 have remained with the project. There were no financial expenses in 1994.

(3) Review article of position-sensitive detectors (H. Hashizume and G. Mcintyre). In view of the continued development and interest in position-sensitive detectors (p.s.d.'s), the publication of a review article was discussed in the Commission to provide crystallographers with information on the advantages and limitations of various detectors, as well as to correct misunderstandings or confusions about some detector properties. Furthermore, detector development is so rapid that crystallographers are not always well informed about available detectors. The standpoint of the review would not be that of detector builders but of users. Our position is not that of a crystallographer who is given a detector and thinking about possible experiments. Rather, we will prepare articles that supply information on up-to-date p.s.d.'s and help crystallographers select the most appropriate detector for his experiment. Dr A. M. Glazer, Editor of J. Appl. Cryst., was very much interested in our project, and agreed to publish any articles in the journal.

An agreement was reached that the review will cover up-to-date integrating and nonintegrating p.s.d.'s for X-rays, neutrons and other particles including electrons. Separate articles would be devoted to different radiations. In addition to 1D and 2D p.s.d.'s, we planned to cover high-count-rate point detectors, which are greatly required at synchrotron sources. Detectors based on different principles and technologies, commercially available or not, would be surveyed. Each article would briefly describe the detector principle (or physics) and construction, but details would be left to references. The properties on which the authors are advised to provide information were discussed. Critical comparisons of different types of detector will be most helpful. Descriptions of typical applications will be illustrated. Overlaps between the articles are inevitable and would not be minimized: evaluations from different viewpoints would be of interest. Exhaustive reference lists will be required in each article. According to this plan, a list of possible contributors was drawn up by March 1994. Soon after, the inaugural issue of the Journal of Synchrotron Radiation was published, which included a review similar to that planned by the Commission. Discussion is under way to work out a new plan.

(4) High Pressure Group (R. J. Nelmes). The principal activities of the High Pressure Group (HPG) in 1993-94 have been the promotion, support and organization of specialist Meetings of various kinds. New experimental techniques on synchrotron and neutron sources, plus the advent of third-generation synchrotrons, are stimulating such an extraordinarily rapid growth in the range and quality of high-pressure crystallography that quite frequent Meetings are needed to keep the community in touch with the latest developments. The Group has been involved, at some level, in the following Meetings during the past year.

1. A Microsymposium on High Pressure Crystallography at the IUCr Congress in Beijing, in August 1993. The Organizing Chairman was Y. Fujii, a member of the HPG.

2. A one-day Workshop on Synchrotron Radiation at High Pressure, in Stony Brook, USA, organized as a Satellite to the Synchrotron Radiation Instrumentation '94 Conference, in July 1994.

3. A Microsymposium on High Pressure Crystallography at the European Crystallographic Meeting (ECM-15) in Dresden, in September 1994. The Organizing Chairman was R. J. Nelmes, Chairman of the HPG, with H. Schulz, Past Chairman of the HPG, acting as Co-chairman.

4. A one-day Workshop on Diffraction at High Pressure, in Dresden, immediately following ECM-15. The organizer was D. Hausermann, a member of the HPG.

5. An International Seminar on Neutron Scattering at High Pressure, held in Dubna, Russia, in October 1994.

Looking to the future, plans are currently well advanced for a three-day International Workshop to be held in Tsukuba, Japan, in March 1995. This is the fourth in a series of HPG Workshops, following previous ones in Munich (1989), Daresbury (1991) and Washington (1992). The organizing Chairman is O. Shimomura, a member of the HPG. Beyond that, it is intended to organize a Workshop in the USA in 1996, in association with the IUCr Congress in Seattle (possibly as an official Satellite Meeting). The HPG is requesting a Microsymposium and a Plenary Lecture at the Congress itself.

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