|The IUCr is an International Scientific Union. Its objectives are to promote international cooperation in crystallography and to contribute to all aspects of crystallography, to promote international publication of crystallographic research, to facilitate standardization of methods, units, nomenclatures and symbols, and to form a focus for the relations of crystallography to other sciences.|
We are sorry to report the passing of David Hoare, a key member of the IUCr staff in the Chester office. David worked in the research and development group, with particular expertise in database and web programming. Many in the worldwide community of crystallographers will know him personally through his prompt, courteous and careful responses to software support queries; many more are unknowingly indebted to him each time they use the World Directory of Crystallographers, purchase goods or services directly through the IUCr website, submit their articles, search or navigate through the beautifully designed new journals website.
His colleagues have lost a highly valued workmate and friend. To his family we extend our heartfelt sympathy: David's strength of character was never more obvious than in his family life. He has been an inspiration to us all.
Waveguides are widely used for filtering, confining, guiding, coupling or splitting beams of visible light. However, creating waveguides that could do the same for X-rays has posed tremendous challenges in fabrication, so they are still only in an early stage of development.
In the latest issue of Acta Crystallographica Section A: Foundations and Advances, Sarah Hoffmann-Urlaub and Tim Salditt report the fabrication and testing of a millimetre-sized chip capable of splitting a beam of X-rays [Acta Cryst. (2016), A72, doi:10.1107/S205327331601144X]. Fork-shaped channels that are only a few tens of nanometres wide and deep are transferred into a silicon wafer using electron-beam lithography and reactive ion etching then enclosed by bonding a second silicon wafer on top. The results of simulations of how the 'parent' beam is split into two 'daughter' beams on passing through the chip were backed up by experimental measurements at the European Synchrotron Radiation Facility, showing that the incident beam is efficiently transported through the chip, neatly split and guided to exits that have precisely controlled (and tunable) spacings. After the daughter beams leave the chip, they interfere, leading to a pattern of vertical stripes just like the pattern obtained from a classical Young's double-slit interference experiment. Interestingly, on close inspection there are fork-like structures within the stripes that originate from discontinuities in the phase of the recombined beam, forming striking features known as phase vortices. Furthermore, from those interference patterns the intensity distribution in the exit plane of the channels is reconstructed, which is found to be in very good agreement to the actual channel design.
This study complements earlier work on two-dimensionally confined channels in silicon in straight and tapered geometries, and paves the way to realizing `X-ray optics on a chip'. Illumination of samples by the two beams could provide some interesting advantages for coherent imaging and opens up the possibility of a new form of nano-interferometer. The authors envisage future development of their beamsplitter to create several daughter beams from the same parent beam, which would allow a single object to be imaged simultaneously by several beams, each from a different direction.
Free Electron Lasers (FELs) are transforming photon science and its various applications. Compared to X-ray pulses from third-generation synchrotron light sources, FEL pulses are 1010 times higher in peak brightness, are 109 times higher in their coherence degeneracy parameters and provide femtosecond-attosecond time resolutions. These extraordinary parameters of FELs have stirred intense development in ultra-fast imaging science.
However, the high repetition rate of X-ray pulses from FEL facilities leads to large volumes of experimental data, whose nature has necessitated the development of new approaches to both data classification and analysis within the specialist groups working with FELs. The latest virtual special issue of Journal of Applied Crystallography (http://journals.iucr.org/special_issues/2016/ccpfel) presents a collection of software that aims to allow a broader community of users to take advantage of the revolutionary new capabilities of FELs. Topics covered include simulation of experiments, online monitoring of data collection, selection of hits, diagnostics of data quality, data management, data analysis and structure determination for both nanocrystallography and single-particle diffractive imaging.
This virtual special issue is published simultaneously with a collection in Nature’s Scientific Data (http://www.nature.com/sdata/collections/xfel-biodata), with articles describing diffraction data obtained on a range of systems with the LCLS. All the data sets are available from the Coherent X-ray Imaging Data Bank (http://www.cxidb.org/), providing a valuable resource for researchers wanting to test software and to develop ideas, tools and procedures to meet challenges with the expected torrents of data from X-ray lasers.
Organic molecules can have a remarkable array of solid forms, including different polymorphs and various multi-component systems, such as salts and co-crystals. The potential diversity of this solid-form landscape presents both opportunities and headaches for the practical use of molecules in solid forms.
Experimental screening of the solid-form landscape can be a time-consuming and expensive process. It is therefore not surprising that over the past 25 years numerous computational methods have been developed to predict crystal structures, providing an alternative or supplement to experimental screening of solid forms and allowing us to explore the solid state of molecules that have yet to be synthesized.
Essentially, the ultimate goal of crystal-structure prediction (CSP) methods is to be able to explore the possible polymorphs, co-crystals, salts, hydrates etc. of a molecule based solely on minimal information such as its two-dimensional chemical diagram.
In the case of organic CSP, progress over the last 15 years has been charted by a series of blind tests of CSP methods that have been hosted by the Cambridge Crystallographic Data Centre (CCDC). Results from the latest study have recently been published in Acta Crystallographica Section B: (2016) Acta Cryst. B72, doi: 10.1107/S2052520616007447.
The sixth blind test of organic CSP methods was held with five target systems: a small nearly rigid molecule, a polymorphic former drug candidate, a chloride salt hydrate, a co-crystal and a bulky flexible molecule. The test has been the biggest to date, with 21 submissions attempting to predict one or more of the five target systems and four submissions re-ranking other predictions with different methods. The range of methods and approaches have shown the development of the field, with progress in the treatment of conformational flexibility in molecules, wider use of ab initio or ab initio-based methods for optimizing and ranking the final structures, as well as more well defined and systematic protocols for performing CSP calculations.
It is with great sadness that we report the death of Hugo Rietveld at the age of 84 after a short illness.
Hugo M. Rietveld was born in The Hague, The Netherlands, on 7 March 1932. After completing Grammar School he went to Australia and studied physics at the University of Western Australia in Perth.
In 1964 he obtained his PhD degree in Physics with a thesis entitled 'The Structure of p-Diphenylbenzene and Other Compounds', a single-crystal neutron and X-ray diffraction study. This investigation was the first single-crystal neutron diffraction study in Australia and was conducted at the nuclear reactor, HIFAR, in Sydney.
In 1964 he became a research officer at the Netherlands Energy Research Foundation ECN at Petten, The Netherlands, and was mainly involved in neutron powder diffraction studies of uranates and other ceramic compounds. After a scientific and managerial career with ECN he retired in 1992, but continued to garner a multitude of awards over the following two decades. These included the Gregori Aminoff Prize in 1995, the Barrett Award in 2003 (pictured), the Order of Oranje-Nassau in 2004, and the EPDIC Award for Distinguished Powder Diffractionists and the Hans Kühl Medal, both in 2010.
The work for which he is best known is the Rietveld Refinement Method, first published in J. Appl. Cryst. (1969), 2, 65-71.
Coherent X-ray diffraction imaging (CXDI) has been used to visualise the internal structures of non-crystalline particles with dimensions of micrometers to sub-micrometers in materials science and biology.
Now X-ray free-electron lasers (XFELs) are leading CXDI to a new levels. X-ray pulses with complete spatial coherence and duration are available at XFEL Facilities such as the Linac Coherent Light Source (LCLS) and the Spring-8 Angstrom Compact Free-Electron Laser. One can routinely collect diffraction data before radiation-damage processes occur and owing to the repetition rate of XFEL pulses, a large number of diffraction patterns can be collected within a short period of time.
A group of scientists [Kobayashi et al. (2016), J. Synchrotron. Rad. 23, doi:10.1107/S1600577516007426] have recently proposed XFEL-CXDI experiments for frozen-hydrated biological specimens at cryogenic temperatures. It has been demonstrated previously that cryogenic technologies enable the storage of biological cells at low temperatures, for instance in medical science. Frozen hydrated cells and cellular organelles keep their functional structures, and are still alive after returning to ambient temperatures.
Cryogenic sample preparation enables the storage of frozen-hydrated specimens in liquid nitrogen for CXDI experiments, scientists can therefore harvest a large numbers of cells and isolated unstable cellular organelles at a desired period of the cells cycle. Moreover, frozen-hydrated specimens are free from adiabatic expansion, bubbling of water inside biological specimens and other phenomena.The scientists have developed membranes, devices and procedures to prepare frozen-hydrated biological specimens and standard specimens for cryogenic XFEL-CXDI experiments. The quality of the prepared specimens was examined through a series of diffraction experiments and based on the results, the researchers discussed future developments in specimen preparation, and the characteristics of frozen-hydrated biological specimens in CXDI experiments.