The work of the Commission during the triennium was conducted entirely by electronic and snail mail. All members are ex officio appointments, as Editors of the Union's journals or volumes of International Tables for Crystallography or as Chairpersons either of the IUCr/OUP Book Series Committee or the Commission on Crystallographic Teaching. The Commission's principal activities have been carried out partly through its internal efforts and partly through the contributions of its sub-committees and working groups.
The Commission records with deep sadness the loss of its longest serving and highly effective member Professor A. J. C. Wilson, who died 1 July 1995 [see obituary in Acta Cryst. (1996), A52, 7-10].
A Working Group on the Expression of Uncertainty in Measurement, consisting of D. Schwarzenbach (Chair), S. C. Abrahams, H. D. Flack, E. Prince and A. J. C. Wilson, was appointed during the triennium. It completed its charge of examining the possible impact of the International Standardization Organization's Guide to the Expression of Uncertainty in Measurement on crystallographic statistical nomenclature with a report entitled Statistical Descriptors in Crystallography. II, which was published in Acta Cryst. (1995), A51, 565-569. Among its major recommendations are replacement of the term estimated standard deviation by either standard uncertainty or 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 to be considered complete only when accompanied by a quantitative statement of the uncertainty of that estimate.
Several membership changes have occurred in the Sub-committee on the Nomenclature of N-Dimensional Crystallography. Its new Chair is Professor T. Janssen, with J. L. Birman, Th. Hahn, V. Koptsik, M. Senechal, D. Weigel, H. Wondratschek, A. Yamamoto and S. C. Abrahams, ex officio, as members and N. D. Mermin of Cornell University and G. Chapuis of the Université de Lausanne as advisors. The Sub-committee is proceeding very systematically in its consideration of the concepts, definitions and terminology of crystallography in N dimensions, achieving consensus 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.
A Sub-committee on Atomic Displacement Parameter Nomenclature, consisting of K. N. Trueblood (Chair) and H.-B. Bürgi, H. Burzlaff, J. D. Dunitz, C. Gramaccioli, H. H. Schulz, U. Shmueli as members and S. C. Abrahams, ex officio, was appointed during the triennium. Its report entitled Atomic Displacement Parameter Nomenclature was completed in record time, was accepted by the Commission, and is now in press [Acta Cryst. (1996), A52, 770-781]. The report considers the confusing and inconsistent terms and symbols in current use for parameters denoting dynamic or static displacements of atoms in crystals. It focuses on individual atomic anisotropic displacement parameters representing atomic motion and possible static displacive disorder, provides clear definitions for these quantities, discusses graphical representations of the Gaussian mean-square displacement matrix and the expressions used if the Gaussian approximation is inadequate, and makes recommendations for symbols and nomenclature.
A Working Group on Phase Transition Nomenclature was also appointed during the triennium. 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. Professor P. Tolédano subsequently resigned and was succeeded by Professor J.-C. Tolédano. R. L. Snyder has also resigned very recently and a ballot on his successor is in progress. 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 interests in phase transitions, the members were elected both for their expertise in the field and their varied scientific backgrounds. In addition, at the Commission's invitation, Professor R. S. Berry was appointed by IUPAP and Professor R. Metselaar by IUPAC as liaisons to the working group.
8 May 1996 S. C. ABRAHAMS, Chairman
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