IUCr 1996 Triennial Report - Commission on Crystallographic Apparatus

During the triennium, the Commission has promoted two major projects: Precision in Lattice Parameters Measurements and Absolute Structure Determination of Light Atom Compounds. This report first describes these projects and then provides a summary of the activities of the high-pressure crystallography group. The project or group leaders are indicated.

(1) Precision in Lattice Parameters Measurements (G. DeTitta and L. Finger)

This is a long-standing, some would say long-suffering, project initiated by the IUCr to do two things. One is to estimate the magnitude of the variance in lattice parameters as reported by structural crystallographers; i.e. those scientists who are primarily reporting structure determinations and for whom the lattice parameters are a necessary ingredient for the calculation of bond distances etc. The group to be surveyed is in particular not those scientists who make their living by the accurate measurement of lattice parameters as an end in itself, such as those in the doped semiconductor business. The other, a purpose added by the Chairman, is to uncover the causes of these variations so that structural crystallographers might become more aware of them and might report more accurate lattice parameters.

In the early stages of the project, we solicited opinions and information from structural crystallographers as to how they determined lattice parameters and as to their opinions on what constituted sufficient data to analyse the raw data coming from their diffraction equipment. The survey also identified a number of scientists willing to participate in a round robin of measurements on crystals to give an experimental estimate to the true variances of reported lattice parameters.

We chose an unknown crystal with a number of considerations in mind. The crystals should be stable in the X-ray beam, have fairly well developed morphology and have lattice parameters that range from small to large. The appearance in the literature of a report on large zeolite crystals made the choice rather simple. The kind gift of these crystals by Professor Juan Garces, University of Toronto, Canada, made it possible to select a suitable number of samples for the round robin. In addition to the unknown, a crystal with well known lattice parameters was chosen to furnish particular information about the state of the diffraction equipment used in the round robin. The ruby spheres given as gifts to those who attended the IUCr meeting in Ottawa were still commercially available at a reasonable price and a quantity of them was purchased from Arcanum in Wisconsin. The zeolite crystals were deliberately chosen because their alignment at the centres of the diffractometers would be non-trivial. No effort was made to make the crystal mount deliberately difficult to centre but no effort was made to ensure that the crystal edges were parallel, or faces perpendicular, to the mount direction. On the other hand, the ruby spheres are small (0.015 mm diameter) and are thus very readily placed at the diffractometer centres.

The bulk of the mounting, screening and analysis of the two kinds of crystal was carried out in the last two years. This is described in the 1995 annual report. As mentioned, the kits are still out in the field, and it is expected to have the results from 30 or more participants in hand by mid-July. It is planned for the organizers to digest the information submitted by the participants and write an interim report on the results. As to the future, we plan to pursue the participants who have yet to make measurements on the kits we sent them. When we have concluded that waiting further will be unrewarding, we will write a report, in the form of an article in Acta Crystallographica, describing the results in full. This work has received financial support from various groups and individuals. The IUCr provided the original funding and there was additional support from the American Crystallographic Association, NIST, and finally from the organizer.

(2) Absolute Structure Determination of Light Atom Compounds (E. Weckert and L. Malakhova)

Purposes. The main purpose of this project is the comparison of different methods for the determination of the absolute structure of light-atom compounds. These are compounds that will be comprised solely from elements from H to O. Depending on the space group of the crystal, the term absolute structure has different meanings. For a chiral molecule in a chiral space group, it is the determination of the absolute configuration. It is the absolute conformation for an achiral molecule in a chiral space group. In a polar space group, the structure will be fixed with respect to the polar direction. And in space groups that have rotoinversion axis =4 and =6 the structure will be fixed with respect to an absolute coordinate system. The significance of the knowledge of the absolute structure, e.g. in pharmacy or for understanding physical properties, is self-evident. The traditional way to fix the absolute structure is the use of anomalous-dispersion effects. The anomalous-dispersion effects of the O and N atoms in very careful experiments using copper radiation are normally sufficient to solve the absolute structure if the ratio of the C to O atoms is nearly one. A comparison of the results obtained in different laboratories on several compounds which vary from a C:O content of 1:1 to O-free compounds with a C:N ratio of 14:1 is planned. It is expected that these comparisons will reveal the limits of what can be achieved with present-day equipment and software. Another important question to solve is down to what O or N content a reliable determination of the absolute structure is possible. It is further planned to compare not only the results of these investigations but also the effort that was necessary, including the equipment and data-processing software that was used. As a result, we expect guidelines of general interest for the regime in which the determination of the absolute structure will be reliable. This should enable other researchers to optimize their effort for an absolute structure determination. As many researchers rely on standard data-processing software, a comparison of different systems might be very important for providing general guidelines. In many laboratories, 2D detectors are meanwhile used also for small-molecule structures. The question we wish to answer is whether the quality of these data compared with four-circle diffractometer data is sufficient to solve the absolute structure. There is another method for the determination of the absolute structure, which is the direct measurement of triplet phases by three-beam interference experiments. The advantage of this method is that it works even in the absence of anomalous-dispersion effects. The disadvantage is the need for crystals of low mosaicity. It is planned to compare the results, reliability and the effort of this method with those that use the breakdown of Friedel's law.

Progress. 1994: The project started. The discussion was entirely dedicated to the selection of suitable test samples. 1995: Purchase of test compounds and distribution among the participants. Start of both kinds of experiment (anomalous dispersion and three-beam diffraction). 1996: Experiments are still going on. First results have been obtained. No comparisons between the results of different laboratories have been made so far.

Results. Only partial results have been obtained so far. The first batch of test crystals, which contains mainly crystals with relatively high oxygen content, has been distributed among the participants. These experiments are not yet finished. The determinations of the absolute structure by three-beam diffraction are nearly finished and have been successful so far. The second batch of crystals, which is more difficult, has not yet been distributed among all the participants. A final comparison of all experimental results can only be carried out if a major part of the data is available.

Future prospects. The project is lasting longer than originally planned. A rough estimate for having a major part of the intensity data sets available would be spring to summer of 1997. For the evaluation of the data and the discussion of the results, we should plan at least six months. As this procedure does not depend on the availability of diffractometers and beam time, it should be much faster than the data-collection part. This means that this project should come to a final conclusion by the end of 1997. Despite the delay, there seems to be no reason for this project not to come to a successful end.

(3) High-Pressure Group (R. J. Nelmes)

The principal activities of the High-Pressure Group (HPG) in 1993-1996 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 HPG has been involved, at varying levels, in the following meetings since the last Congress.

(i) A one-day Workshop on Synchrotron Radiation at High Pressure, Stony Brook, USA, organized as a satellite to the Synchrotron Radiation Instrumentation '94 conference, in July 1994.

(ii) A Microsymposium on High-Pressure Crystallography at the European Crystallography Meeting (ECM-15) in Dresden, 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.

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

(iv) An International Seminar on Neutron Scattering at High Pressure, held in Dubna, Russia, October 1994. The Proceedings have been published in High Pressure Research, Volume 14.

(v) The major event was a three-day international Workshop held in Tsukuba, Japan, March 1995. This was the fourth in a series of HPG Workshops, following previous ones in Munich (1989), Daresbury (1991) and Washington (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.

(vi) 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 USA, Japan, France, Germany, UK, Russia, Switzerland, Canada, The Netherlands and Spain, 8 other contributed talks, and 35 poster presentations. It is expected to prove 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.

25 June 1996 H. HASHIZUME , Chairman

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