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IUCr 1993 Triennial Report - Commission on Crystallographic Apparatus

Over the past triennium, the Commission on Crystallographic Apparatus has had as its primary aim the solution of problems associated with the calculation and measurement of the fundamental parameters of X-ray scattering (f ', f '' and s). It has been actively involved in theoretical and experimental determinations of f ' and f '', the measurement of X-ray absorption coefficients, and the creation of standards and criteria for XAFS experiments. In addition, it has been interested in the precise measurement of single-crystal lattice parameters and the development of criteria for correctly specifying the properties and performance of two-dimensional detectors. Its High Pressure Group has been involved in the organization of conferences and Workshops. Considerable progress has been made in most aspects of its work. Now that many of its projects have reached a satisfactory conclusion, the Commission is planning to broaden its activities, and will concentrate on the techniques of measurement in both X-ray and neutron diffraction, where these two fields have problems in common.

1. The X-ray Attenuation Project (D. C. Creagh). This project reached its conclusion with the publication of International Tables for Crystallography Volume C. The tables of X-ray absorption and related tables on X-ray spectroscopy and the dispersion corrections which were presented in that volume have been compared with existing theoretical and experimental data sets [Creagh (1990). Nucl. Instrum. Methods, A295, 417-434; Creagh (1992). Proceedings of the International Conference on Anomalous Scattering, Malente; Creagh (1992). National Seminar on Recent Trends in Photon-Atom Interactions, Karnatak]. Work is proceeding on the production of parameterized fits to the X-ray attenuation data and the generation of MS DOS programs for computing f' and f ''.

2.The Single-Crystal Lattice-Parameter Project (G. T. De Titta). This project has made slow progress this triennium. The requisition of ruby spheres of sufficiently high perfection has not been easy, and each sphere must be mounted and tested prior to distribution to laboratories participating in the project. This project has two labour-intensive phases: specimen preparation and selection, and data collation. It is recommended that funding be provided for assistance in both phases of the project.

3.The Accuracy in XAFS Project (D. C. Creagh, H. Oyanagi and R. Frahm). We have been involved in the development of standards and criteria for the performance, the data reduction and subsequent publication of the results from XAFS experiments. We have been concerned with the improvement of the status of XAFS research. The results of the first two Workshops which were concerned with the establishment of standards and criteria for XAFS research have been published [Lytle, Sayers & Stern (1989). Physica (Utrecht), B152, 701-721; (1991). X-ray Absorption Fine Structure, edited by S. S. Hasnain, pp. 751-771. London: Ellis Horwood]. The third Workshop was concerned with the development of an International Short Course on XAFS. The instructional modules for this course are now being written. The section on X-ray optics has been written by D. C. Creagh. In addition, an XAFS data file has been set up in collaboration with the NNDC at Brookhaven, USA. It is anticipated that this data file will be as valuable to XAFS users as the JCPDS file has been to users of X-ray powder diffraction. Largely because of our involvement, the XAFS body has become a formal entity. It is now the International XAFS Society (IXS). Discussions with its President, D. Sayers, are proceeding to ascertain what kind of relationship might exist between the IUCr and the IXS.

4.Evaluation of Two-Dimensional Detectors. Some progress has been made on this project. Whether or not this project can proceed further depends on the response of IUCr members to papers on area detectors to be given in our Open Commission Meeting at the forthcoming Beijing Congress.

5.The Absolute Structure Determination of Light-Atom Compounds (L. Malakhova). Professor Malakhova has established that sufficient laboratories are interested in this project to make it viable. It has been decided to proceed with this project and funding for it has been requested in the next triennium.

6.The High-Pressure Group Committee (R. Nelmes). This Committee has organized two highly successful Workshops, the first at Daresbury in conjunction with the Synchrotron Radiation Instrumentation Conference (Chester, England, 1991) and the second in collaboration with the Commission on Powder Diffraction (Washington, USA, 1992). Its plans for its Microsymposium at the Beijing Congress are well advanced. The High-Pressure Group was formed from the remnants of the Commission on Crystallographic Studies at Controlled Pressures and Temperatures, which was disbanded at the Perth Congress. Since its formation, the group has been vigorous in its activities and has greatly expanded its membership. If its work continues to prosper the IUCr ought to consider reforming it as a High Pressure Commission.

The Commission is also considering the possibility of becoming involved in a project to measure X-ray wavelengths, which has been proposed by R. Deslattes (NIST). Crystallographers have long been aware of deficiencies in the tables of X-ray wavelengths printed in International Tables for X-ray Crystallography Volume III, which have been reprinted in International Tables for Crystallography Volume C. The resolution in discrepancies in these tables is of great importance to all crystallographers.

10 April 1993                                D. C. CREAGH, Chairman

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