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IUCr 1994 Report - Commission on Crystallographic Nomenclature

The Commission's work in 1994, as in previous years, was continued partly through its internal efforts and in larger part through the important contributions made by its Subcommittees and Working Groups, with communications being conducted primarily by electronic means. Membership in the Commission is entirely ex officio, by virtue of being an Editor of an IUCr journal or of an individual volume of International Tables for Crystallography; the Chairpersons of the IUCr/OUP Book Series Committee and the Commission on Crystallographic Teaching are also ex officio members.

The Working Group on the Expression of Uncertainty in Measurement, with membership as given in Acta Cryst. (1995), A51, 211 and with D. Schwarzenbach as Chair, completed its Report entitled Statistical Descriptors in Crystallography. II during the year. The Report was accepted by the Commission in September and by the Executive Committee in December 1994 and is now in press [Acta Cryst. (1995), A51, 565-569]. Among the Report's major recommendations are replacement of the term estimated standard deviation by standard uncertainty or by combined standard uncertainty in statements of the statistical uncertainties of data and results. The report presents the concepts of standard uncertainty as developed by the International Organization for Standardization, and of Type A and Type B evaluations of standard uncertainties. The result of a measurement is only an estimate of the value of the measurand and is now considered complete only when accompanied by a quantitative statement of the uncertainty of that estimate.

Several membership changes have occurred in the Subcommittee on the Nomenclature of N-Dimensional Crystallography. Its new Chair is Professor T. Janssen, with J. L. Birman, Th. Hahn, V. Kopstik, M. Senechal, D. Weigel, H. Wondratschek and A. Yamamoto as members and S. C. Abrahams as an ex officio member [see also Acta Cryst. (1995), A51, 211]. The Subcommittee is proceeding very systematically in its consideration of the concepts, definitions and terminology of crystallography in N dimensions, achieving concensus as it progresses. Definitions of higher-dimensional point groups, crystal classes, holohedry and space groups, for example, have been considered as have rules for ordering four-dimensional families and symbols for point groups and their transformations. The task is large and progress is expected to be slow but deliberate.

Two new Subcommittees were established by the Commission in 1994. Professor K. N. Trueblood was elected Chair of the new Subcommittee on Atomic Displacement Factor Nomenclature, with H.-B. Bürgi, H. Burzlaff, J. D. Dunitz, C. Gramaccioli, H. H. Schulz and U. Shumeli as members with S. C. Abrahams as an ex officio member. The Subcommittee's charge is to examine the merits of adopting a uniform approach to reporting atomic displacement factors in structural papers and with making such recommendations as may be appropriate. The motivation for establishing this Subcommittee sprang from the widespread confusion in the literature regarding the use of such terms as U, B and Beta and the different ways in which they have sometimes been defined. A thorough treatment has been developed of the displacement parameters based on the Gaussian approximation, and also those needed when that approximation is not valid. A final report is expected in the first half of 1995.

A Working Group on Phase Transition Nomenclature was also formed in 1994. Professor P. Tolédano was elected Chair, with A. M. Glazer, E. Parthé, R. L. Snyder and Th. Hahn as members and S. C. Abrahams ex officio. The Working Group is charged with studying the multiple nomenclature in current use for naming the sequence of phases that a material may form as a function of temperature or pressure and with making whatever recommendations may be appropriate. In view of the interdisciplinary interest in phase transitions, the members were selected both for their expertise in the field and for their varied scientific backgrounds. In addition, at the Commissions's invitation, Professor R. S. Berry was appointed by IUPAP and Profesor R. Metselaar by IUPAC as liaisons to the Working Group. The Chair initiated discussions by providing a valuable and comprehensive overview of the primary kinds of phase transitions and transformations. Travel by members has slowed the rate of progress, but this is expected to increase in 1995.

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