Acta Crystallographica Section A最新文献

It was shown in a previous paper [Eon (2004). Acta Cryst. A60, 7-18] that the topological density of a periodic net can be calculated directly from its cycles figure, a polytope constructed from those cycles of the quotient graph of the net that are associated with its geodesic lines. It may happen that these lines generate a grid pattern forming a supercell, a phenomenon that was not considered in the former derivation of the formula but is common for lattice nets. An adjustment of the expression is proposed to this effect and applied to the square and hexagonal lattice nets as well as to the 13 families of cubic lattice nets.

Although neutron diffraction was first observed using radioactive decay sources shortly after the discovery of the neutron, it was only with the availability of higher intensity neutron beams from the first nuclear reactors, constructed as part of the Manhattan Project, that systematic investigation of Bragg scattering became possible. Remarkably, at a time when the war effort was singularly focused on the development of the atomic bomb, groups working at Oak Ridge and Chicago carried out key measurements and recognized the future utility of neutron diffraction quite independent of its contributions to the measurement of nuclear cross sections. Ernest O. Wollan, Lyle B. Borst and Walter H. Zinn were all able to observe neutron diffraction in 1944 using the X-10 graphite reactor and the CP-3 heavy water reactor. Subsequent work by Wollan and Clifford G. Shull, who joined Wollan's group at Oak Ridge in 1946, laid the foundations for widespread application of neutron diffraction as an important research tool.

A tribute to W. L. Bragg by his younger daughter is presented.

The cross-correlation function between the target and a model electron density, denoted as the C map, has been crystallographically characterized. In particular, a study of its interatomic vectors and of their relation with the Patterson vectors has been undertaken. Since the C map is not available during the phasing process, the C' map, its centric modification, is considered. It may be computed at any stage of the phasing process and shows properties that are very useful for the crystal structure determination process. It has been combined with the implication transformation method and with vector-superposition techniques for performing the Patterson deconvolution and obtaining an initial model for dual-space recycling. While Patterson methods are traditionally considered to be more efficient for structures containing heavy atoms, the C map extends their potential to light-atom structures (i.e. containing atoms not heavier than O).

W. H. Bragg arrived in Australia in 1886 as Head of the Mathematics and Physics Departments at the University of Adelaide. His son, W. L. Bragg, grew up in Adelaide and graduated from the Physics Department. Many years later I graduated from the same department and had the opportunity to share Lawrence Bragg's recollections of life in Adelaide. As well as touching on the 'Adelaide' connection, this report briefly reviews Bragg's critical role in encouraging, supporting and establishing the field of large-molecule crystallography.

An extract from the obituary for Sir Lawrence Bragg by M. F. Perutz [Nature (London), (1971), 233, 74-76] is given.

An algorithm is presented which determines the equivalence of two settings of a (3 + d)-dimensional superspace group (d = 1, 2, 3). The algorithm has been implemented as a web tool findssg on SSG(3+d)D, providing the transformation of any user-given superspace group to the standard setting of this superspace group in SSG(3+d)D. It is shown how the standard setting of a superspace group can be directly obtained by an appropriate transformation of the external-space lattice vectors (the basic structure unit cell) and a transformation of the internal-space lattice vectors (new modulation wavevectors are linear combinations of old modulation wavevectors plus a three-dimensional reciprocal-lattice vector). The need for non-standard settings in some cases and the desirability of employing standard settings of superspace groups in other cases are illustrated by an analysis of the symmetries of a series of compounds, comparing published and standard settings and the transformations between them. A compilation is provided of standard settings of compounds with two- and three-dimensional modulations. The problem of settings of superspace groups is discussed for incommensurate composite crystals and for chiral superspace groups.

The influences of Lawrence Bragg and Max Perutz are evident in the contemporary emphasis on 'structural enablement' in drug discovery. On this occasion of the centenary of Bragg's equation, his role in supporting the earliest structural studies of biological materials at the Cavendish Laboratory is remembered. The 1962 Nobel Prizes for the structures of DNA and proteins marked the golden anniversary of the von Laue and Bragg discoveries.

The Nobel Committees have to follow the nominations submitted for a specific year. During the early phase of X-ray crystallography, a limited number of scientists were active. In 1914 Max von Laue and William Henry Bragg were both nominated and could have been awarded a joint Nobel Prize. However, a member of the Nobel Committee for Physics, Allvar Gullstrand, was well aware of the activities in the field and strongly recommended that only von Laue should receive the prize since a main contributor, William Laurence Bragg, was not nominated. Next year, when the First World War had started, there were few nominations, but now both Braggs, father and son, were nominated. Gullstrand was very pleased and recommended them both for the 1915 Nobel Prize in Physics. The rest of the committee agreed and this then became the decision of the Royal Academy for Sciences, Stockholm.

This paper recounts the atmosphere in the Cavendish Laboratory during Lawrence Bragg's triumphant final years there through the eyes and the work of a young research student, and hence reflects some measure of Bragg's personality. The opportunity is taken to deal in detail with Bragg's contribution to our understanding of crystal plasticity, which is seldom described, being overshadowed by his many superb contributions to the determination of crystal structure. Bragg produced in 1940-1942, through his development of the bubble model of a crystal structure, the first demonstration of how crystal dislocations move. His suggestion of the use of microbeams led rather directly to the development of modern thin-film transmission electron microscopy.