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IUCr 1995 Report - Commission on Crystallographic Nomenclature

The work of the Commission in 1995 was accomplished, as in prior years, both internally and through the major contributions made by its sub-committees and working groups; most communications used electronic means. Membership of the Commission is entirely ex officio, by virtue of a primary IUCr responsibility closely related to crystallographic nomenclature.

The Commission reports 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. Dr E. Prince was welcomed as the new Editor of International Tables for Crystallography, Volume C. The report of the Working Group on the Expression of Uncertainty in Measurement, entitled Statistical Descriptors in Crystallography. II, was published in Acta Cryst. (1995), A51, 565-569. The report presents the concepts of standard uncertainty and of Type A and Type B evaluations of standard uncertainties as developed by the International Organization for Standardization (ISO) and recommends replacement of the term estimated standard deviation by standard uncertainty or combined standard uncertainty in statements of the statistical uncertainties of data and results. The conclusion, long held by the IUCr, that the result of a measurement is complete only when accompanied by a quantitative statement of the uncertainty in that measurement is now fully recognized by the ISO, with legal consequences.

The Sub-committee on the Nomenclature of N-Dimensional Crystallography, see Acta Cryst. (1996), A52, 91-124 for membership, has elected N. D. Mermin of Cornell University, USA, and G. Chapuis of the Université de Lausanne, France, as advisors. Consideration of the concepts, definitions and terminology of crystallography in N dimensions has continued. A wide range of nomenclature views has emerged and the Sub-committee has decided to confine its attention to areas in which agreement is feasible rather than pursue a more general approach. The former includes symbols for point-group transformations, systems ordering and geometric crystal class notation in four dimensions and preferred settings and cell choices to the sixth dimension.

The Sub-committee on Atomic Displacement Parameter Nomenclature, see Acta Cryst. (1996), A52, 91-124 for membership, completed its report entitled Atomic Displacement Parameter Nomenclature in record time under the able chairmanship of K. N. Trueblood. The report was accepted by the Commission on 20 December 1995 and immediately submitted to the IUCr Executive Committee for approval [reports of the Commission's sub-committees and working groups accepted by the Commission and Executive Committee become IUCr nomenclature policy and are followed by all publications of the Union]. 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 [see Acta Cryst. (1996), A52, 770-781].

The resignation of P. Toledano, Chair of the Working Group on Phase Transition Nomenclature, see Acta Cryst. (1996), A52, 91-124 for membership, was received with regret during the year. Professor J.-C. Tolédano was elected Chair to succeed him. 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.

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