|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.|
Crystal structure analysis began in 1912 with the first papers of William Lawrence Bragg. Just three years later, in 1915, W. L. Bragg and his father, William Henry Bragg, were jointly awarded the Nobel Prize in Physics for their work in formulating the relationship between a crystal’s atomic structure and its X-ray diffraction pattern. Their work has revolutionised our understanding of the structure of matter ranging from minerals, pharmaceutical materials and catalysts to DNA, proteins and viruses.
To commemorate 100 years since the Braggs’ Nobel Prize award for the first use of X-rays to determine a crystal structure, we will aim to publish 100 papers in IUCrJ. The first 100 articles submitted to IUCrJ using the voucher code below will be published as platinum open access (i.e. there will be no charges for open-access publication).
Voucher code: dis-63gwUYye619PkvwixPj
The year 2014 was declared the International Year of Crystallography by the United Nations to raise awareness in the world at large about the importance of crystallography in the modern world. Through an incredible variety of activities worldwide, that objective has been brought to life for thousands of schoolchildren, university students and the general public. Many events have been sponsored by UNESCO and the IUCr, supported by industrial and academic sponsors, and made real by a host of willing volunteers. They have ranged from science fairs for schoolchildren, through professional-level OpenLab workshops and training sessions, to international summit meetings to shape future policy. They have ignited a fire in the new generation, most especially in parts of the developing world where structural science is still an infant science. Crystallography now matters ... more!
To build on these successes, the IUCr is embarking on an ambitious set of new initiatives to ensure that this newly kindled flame does not go out. More OpenLabs will be commissioned; more sustained efforts will be made in capacity building; more effort will go into public outreach activities.
A launch conference, entitled "Crystallography for the next generation: the legacy of IYCr", will be held at the prestigious venue of the Hassan II Academy of Science and Technology of Morocco in Rabat on 22-24 April 2015. The "IYCr legacy" conference will be held in lieu of a "Closing" ceremony for the IYCr. Register here to attend free of charge.
The IUCr is pleased to announce the launch of the IYCr Legacy Fund, a fund established to enable the continuation of many initiatives successfully launched during IYCr2014. The aim of the IYCr Legacy Fund is to support the following:
“Of course, a lasting legacy from the International Year of Crystallography is not just related to outreach activities taking place during the Year,” said Peter Strickland, Executive Managing Editor, IUCr. “It is also about creating lasting initiatives in our schools, colleges, universities and other places of learning to continue the message about the importance of crystallography. We want to see a nation of youngsters being inquisitive about crystallography and governments enabling schools and colleges with opportunities to teach crystallography alongside the core science disciplines currently featured. This can’t happen overnight; however, with the launch of the IYCr Legacy Fund we stand a real chance to accomplish these goals, by continuing many of the initiatives launched during the Year.”
To donate to the IYCr Legacy Fund, please go here.
New structural studies of the superficially simple ammonium carbonate monohydrate could shed light on industrial processes, biochemistry and even the interstellar building blocks of life.
Researchers in Spain and the UK have used Laue single-crystal diffraction methods with pulsed neutron radiation at 10 and 100 Kelvin to obtain a crystal structure of ammonium carbonate monohydrate and have corroborated their findings with X-ray powder diffraction data obtained at 245 to 273 K, Raman spectra (80-263 K) and density functional theory calculations of the electronic structure and phonon spectrum.
Seemingly simple molecules are of fundamental interest as their apparent lack of complexity and limited number of atoms should make them more amenable to structural studies and reduce the size of the data stream that needs to be processed in computations of their properties. One such compound ammonium carbonate monohydrate, (NH4 )2CO3.H2O, was first described by Sir Humphrey Davy in 1800 but despite its purported simplicity it remains something of an enigma, its shifting phases, thermal expansion and hydrogen bonds, giving even the most determined theoretical chemist problems in explaining the properties of the compound.
Dominic Fortes of the University of London and colleagues at University College London, the Instituto de Ciencia de Materiales de Madrid, Spain, the ISIS Facility at the Rutherford Appleton Laboratory in Didcot, UK and the University of Bristol, UK explain how this compound is, at its simplest a ternary system involving a carbon dioxide, an ammonia and a water unit. A trio important in industrial chemistry as well as in many biological reactions.
As the crystalline material, the compound exists in various forms although since its discovery there have been conflicting reports through the years regarding the correct composition of the solid. Even as recently as 1992, the tendency of phase mixtures to crystallize ambiguously is common and some researchers have questioned the veracity of the formula given for a commercially available product. A sample from one supplier tested by the researchers proved to be something entirely different - ammonium carbamate.
However, Fortes and colleagues suggest in a paper [Fortes et al. (2014). B70, 963-972; doi: 10.1107/S205252061402126X] that they have new definitive evidence of the nature of ammonium carbonate monohydrate. The team used Laue single-X-ray powder diffraction data measured from 245 to 273 K, Raman spectra measured from 80 to 263 K and an athermal zero-pressure calculation of the electronic structure and phonon spectrum carried out using density functional theory (DFT).
"We find no evidence of a phase transition between 10 and 273 K," the team reports, "above 273 K, however, the title compound transforms first to ammonium sesquicarbonate monohydrate and subsequently to ammonium bicarbonate."
The chemical cousins of the rather innocuous ammonium carbonate monohydrate, including the toxic ammonium oxalate and the explosive ammonium chlorate and ammonium nitrate have been much better studied despite their obvious drawbacks as useful laboratory reagents when compared to the rather innocuous ammonium carbonate monohydrate. It is curious, the team suggests, that there has been so little interest in the compound's structure and properties. This is also true given that carbon dioxide, ammonia and water are apparently so common in interstellar, cometary and planetary ices and may have a role to play in explaining cosmic chemistry and perhaps even the building blocks of life on Earth and putatively elsewhere in the universe.
"The next step is to measure other physical properties of the carbonate and related compounds," Fortes told us. "In due course, we'll do high-pressure experiments on ammonium carbonate to determine its compressibility and to look for high-pressure polymorphs."
Electrons diffract in the same way as X-rays and neutrons, except that the electron wavelength is very small and the electron scattering cross-section is correspondingly much larger, about a million times that of X-rays. Since electron diffraction was discovered, transmission electron diffraction and the related electron imaging have developed into powerful tools for the analysis of defects, microstructure, surfaces and interfaces in a broad range of materials, so why haven’t more structures been solved with high-energy electrons?
The short answer lies in electron dynamic diffraction: the same strong interaction between electrons and matter that gives rise to large electron scattering cross sections also gives rise to strong multiple scattering. A general method for solving unknown crystal structures using dynamic diffraction intensities has yet to be developed.
In a topical review two researchers [Midgley, P. & Eggeman, A. (2015). IUCrJ, 2, 126-136; doi:10.1107/S2052252514022283] describe progress made in the technique of precession electron diffraction (PED).
Using PED intensities, crystal structures can be solved by a combination of phasing and structure refinement.
The quality of electron diffraction data, as well as speed of acquisition, is increasing rapidly; thus in the not too distant future we can expect more identifications of new structures and their solutions.
[Zuo, J. M. & Rouviere, J. L. (2015). IUCrJ, 2, 7-8; doi:10.1107/S2052252514026797]
Photocrystallography is a rapidly developing technique that involves the determination of the full three-dimensional structure of a molecule or array material, using single-crystal X-ray diffraction techniques, while the molecular components are in a photoactivated metastable or short-lived high-energy state. The photoactivation is usually achieved by irradiating the single crystal with a laser or a set of LED lights in the ultra-violet or visible wavelength range.
To date photocrystallography has been used to study the structures of metastable linkage isomers of transition metal coordination complexes, and to investigate the changes in coordination geometry in mononuclear complexes.
In a complex that undergoes linkage isomerism, the compound contains one or more ligands that are capable of bonding to the metal centre in more than one way. At present, investigations at short lifetimes remain the domain of fast spectroscopy. Thus a combination of photocrystallography and time-resolved spectroscopy allows the evolution of structure with time to be evaluated.
The paper by Casaretto et al. [(2015). IUCrJ, 2, 35-44; doi:10.1107/S2052252514023598] provides new insights into the photactivated linkage isomerisation process and sets the benchmark for further studies that will lead to the development of electronic devices based on these materials.
[Raithby, P.R. (2015). IUCrJ, 2, 5-6; doi:10.1107/S2052252514026980]
There are many important materials which do not form nice single crystals for X-ray diffraction experiments, such as proteins and other biomaterials. When the crystal size is too small then powder diffraction is normally the preferred method of analysis. Until now the minimum size of crystal needed to produce three-dimensional single-crystal data had been dependent on the instrument in operation; now, however, using serial micro crystallography, Ayyer et al. [(2015). IUCrJ, 2, 29-34; doi:10.1107/S2052252514022313] have shown that improvements can still be made from changes to the software and algorithms used.
When a sample diffracts well as a powder but does not grow larger crystals, serial micro crystallography will be the method of choice. This new way to obtain three-dimensional data could bring far more complex structures into reach for powder diffraction.
[Wright, J. P. (2015). IUCrJ, 2, 3-4; doi:10.1107/S2052252514026803]
Although crystal structure determination by means of X-ray diffraction has had a huge scientific impact over the last 100 years, it still requires the solution of the crystallographic phase problem. This problem arises because although the intensities of the diffracted X-rays can be measured, direct measurement of their relative phases is still only rarely practicable. Small-molecule crystal structures are usually solved by the use of probability relationships involving the phases of the stronger reflections, i.e. direct methods, or more recently by Fourier transform methods such as charge flipping.
SHELX is a system of nine programs for the solution and refinement of crystal structures against X-ray and neutron diffraction data.
The first version of SHELX was written around 1970 and officially released in 1976. Using data compression written specially for the purpose, it proved possible to pack the 5000 lines of FORTRAN, five test data sets and a little FORTRAN program to unpack the rest, into one box of 2000 punched cards. This greatly facilitated its distribution by post (email, internet etc. had not yet been invented) and enabled the program to spread around the world. The current version of SHELX has about 9000 registered users in about 90 countries and may be obtained, together with documentation and tutorials etc., via the SHELX homepage.
In 2008 Sheldrick published a review paper [Sheldrick, Acta Cryst. (2008), A64, 112–122] about the system, noting that it might serve as a general literature citation whenever any of the SHELX programs were used. The paper has since received no fewer than 38,000 citations making it the highest-ranked paper published in the last two decades, according to a recent study that appeared in Nature [Van Noorden et al. Nature, (2014), 514, 505-553].
This latest addition to the SHELX program [Sheldrick, Acta Cryst. (2015). A71, 3-8; doi:10.1107/S2053273314026370] employs a novel dual-space algorithm to solve the phase problem for single-crystal reflection data expanded to the space group P1. Missing data are taken into account and the resolution extended if necessary. In testing, SHELXT has already solved many thousands of structures with a high success rate.
A recent upgrade to the SHELXL refinement program is also available [Sheldrick, Acta Cryst. (2015), C71, 3-8; doi:10.1107/S2053229614026540].
In a recent interview for the IUCr the author, commenting on why he thought the SHELX programs are so well received, said, “the programs are extremely robust and adopt a strict 'zero dependencies' philosophy, i.e. no third-party libraries, DLLs, environment variables etc. are required to run them”.