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IUCr 1995 Report - Commission on Crystallographic Apparatus

During 1995, the Commission continued promoting its two major projects: Absolute Structure Determination of Light-Atom Compounds and Precision Lattice-Parameter Determination. Activities also continued in the field of position-sensitive X-ray detectors and high-pressure crystallography.

(1) Absolute Structure Determination of Light-Atom Compounds (E. F. Weckert and L. Malakhova)

The project aims to compare different X-ray methods for the determination of the absolute structures of light-atom compounds. The group decided to concentrate on (i) exploitation of anomalous-dispersion effects and (ii) three-beam diffraction, and tried to establish the limits of the applicability of these methods as a function of radiation, content of anomalous-scattering atoms and crystal quality. It also aims to find not only the limits of what is possible but also the limits of what can be easily achieved in a routine structure determination, including a comparison of different software packages. The following sample compounds were selected, which include different contents of anomalous-scattering atoms: sucrose, C₁₂H₂₂O₁₁ (space group P2₁); L-asparagine monohydrate, C₄H₈N₂O₃.H₂O (space group P2₁2₁2₁); benzil, C₁₄H₁₀O₂ (space group P3₁21); BN (space group F-4m); urotropin, C₆H₁₂N₄ (space group F-4m); benzophenone, C₁₃H₁₀O (space group P2₁2₁2₁); C₁₄N₂O (space group P2₁2₁2₁); C₂₈N₂ (space group P2₁2₁2₁). One problem here was to find samples with very small anomalous-dispersion effects in large quantities.

We decided to start with compounds of high oxygen content and try the more difficult compounds later. The first batch of crystals was distributed among interested participants and experiments are on the way or have finished. The second batch of compounds will be distributed soon. Three-beam interference experiments have successfully been performed with nearly all of the compounds (except sucrose and urotropin). As this method works without anomalous-dispersion effects, the main problem is crystal quality. The participants are listed below with their roles: Professor P. Beurskens, Nijmegen, The Netherlands, numerical analysis of experimental data; Professor S. L. Chang, Taiwan, numerical evaluations of measured three-beam interference profiles; Dr Z. Dauter, York, UK, anomalous-dispersion-based measurements using image-plate detectors and numerical evaluation of experimental data; Dr J. Grochowski, Cracow, Poland, anomalous-dispersion-based measurements and numerical evaluation of data sets; Professor A. Kirfel, Würzburg, Germany, numerical analysis of experimental data; Professor C. Kratky, Graz, Austria, anomalous-dispersion-based measurements and numerical evaluation of data sets; Dr L. Malakhova, Moscow, Russia; Dr D. Watkin, Oxford, UK, anomalous-dispersion-based measurements and numerical evaluation of data sets; Dr E. Weckert, Karlsruhe, Germany, three-beam interference measurements and anomalous-dispersion-based measurements, data analysis.

(2) Precision and Accuracy of Lattice Parameter Determination (G. DeTitta and L. Finger)

After much delay, ten `kits', each containing five ruby spheres and five zeolite crystals, were fabricated by M. J. Kirisits. The crystals were mounted on the tips of X-ray glass capillaries using a minimum of fast-setting epoxy, the capillaries were set in machined brass pins and each of the crystals was subjected to preliminary diffraction analysis to assess quality and to determine approximate cell parameters and orientation matrices. Over 120 crystals were screened for a suitable group of 100 representatives for the kits. A good deal of thought and work went into the design and construction of the packaging for the kit. Given that each kit would be shared among five to seven participants, there would be ample opportunities for postal mishap. The containers were inexpensive fishing-tackle boxes. Into each was placed a handmade wooden frame, fitted out to hold the ten pins securely with nylon lock screws, ten inverted glass vials to protect the crystals and two locking braces to keep the vials from moving. The wooden frame was dimensioned to fit securely and snugly in the fishing-tackle box. Test flights of the boxes indicated that crystals would survive a fall of 2m. The tackle boxes were then placed in sturdy cardboard shipping containers filled with packing foam.

Manuals were written to instruct the participants in the use of the kits. In addition to the preliminary information concerning the orientation matrices of the crystals, there were instructions on the treatment of the ruby spheres. The idea was to ensure that information necessary to evaluate the instrumental variances in the zeolite lattice parameters would come out of the ruby-sphere measurements. References to the literature were included, as well as definitions of the various orientation matrices in use on various diffractometers.

The original list of participants was contacted again to check their continued interest in participation in the project. The kits were mailed at the end of the summer (August, September 1995) to the first ten participants. These participants in turn mailed their kits to various other participants. To date, we have received results from 24 participants. A recent letter was sent to all participants to find out where the remaining kits are (two have been returned).

The plan is now for the data to be interpreted by the organizers. The results of the ruby measurements will be used to determine the alignment of the diffraction equipment and assess its contribution to the variance in the zeolite crystal parameters. A brief report may be made to the Commission at Seattle.

(3) Review on Position-Sensitive Detectors (G. McIntyre)

We made a proposal for a series of review articles on the state of the art of the development of position-sensitive detectors and their application to crystallography using X-rays, neutrons and electrons. While this was intended to indicate important new developments, especially at third-generation synchrotron sources and larger neutron facilities, we also believed that an important role of the Commission was to make recommendations for conventional diffractometry at smaller laboratories and universities.

Shortly after the proposal was made, several extensive overviews of X-ray position-sensitive detectors appeared in the Journal of Synchrotron Radiation, which rendered the first objective of our series unnecessary for the moment. Since then we have mainly observed developments to await the appropriate time for a new review. This should be made soon in view of the much increased use in the last two years of image plates and CCD-based position-sensitive detectors for conventional X-ray crystallography in smaller laboratories and universities and the application of image plates to quasi-Laue methods and the promising results for neutron diffraction from microstrip gaseous proportional counters in neutron diffraction.

In the meantime, we wondered if a future project of our Commission could concern guidelines for graphical user interfaces for diffractometer control. The advent of fast, and relatively cheap, workstations has seen a dramatic change in the way in which we drive crystallographic instruments, and in the visual interface to the control program. The Motif guidelines of the Open Software Foundation are followed by many, but not all, developers of graphical diffractometer control programs. Is there a need, or desire, for additional guidelines for (powder and single-crystal) diffractometer control programs so that all interfaces have a similar look and feel? We believe that there are now a sufficient number of interfaces in the different main areas of crystallography that such guidelines could be established.

(4) High-Pressure Crystallography (R. J. Nelmes)

The major event was a three-day international Workshop held in Tsukuba, Japan, in March 1995. This was the fourth in a series of HPG Workshops, following previous ones in Munich, Germany (1989), Daresbury, UK (1991) and Washington, USA (1992). The organizing Chairman was O. Shimomura, a member of the HPG. About 80 participants from UK, Germany, France, Russia, China, USA and Japan attended. The programme began with a technical session on new optics, detectors and high-pressure techniques for diffraction experiments. Then followed sessions on data collection using diamond-anvil cells and large-volume cells, and data analysis by the Rietveld and maximum-entropy methods. The Proceedings will appear shortly in High Pressure Research.

During the past year, members of the HPG have also been heavily involved in preparations for the 1996 Congress in Seattle, where there is to be a substantial high-pressure programme comprising two Keynote Lectures and six Microsymposia over the first three days of the Congress. All current members of the HPG are chairing or co-chairing sessions and several are giving talks. There are to be 39 invited speakers from the USA, Japan, France, Germany, the UK, Russia, Switzerland, Canada, The Netherlands and Spain, 8 other contributed talks, and 35 poster presentations. It is expected to be a major meeting, celebrating the vitality and exciting future prospects of the field.

It has been a long-term objective of the HPG to raise the profile of high-pressure crystallography within the IUCr - particularly following the demise of the previous Commission on High Temperature and Pressure - and we are grateful to the Congress organizers for their willingness to accommodate our plans for Seattle so generously. All the Chairs and Co-Chairs of the high-pressure sessions have worked hard to create a programme that justifies this support.

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Updated 17th October 1997

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