Recently in Structure of crystalline and non-crystalline materials Category

Coherent X-ray diffraction imaging is one of a number of recently developed lens-less microscopic techniques giving 2D real space structure when combined with phase retrieval data processing. A team in Shandong University in China has recently published an interesting observation of intact unstained magnetotactic bacteria. It was confirmed that the reconstructed images give some intercellular structures, such as nucleoid, polyβ-hydroxybutyrate granules, and magnetosomes, which have been identified by electron microscopy. The team was also successful in quantification of the density, i.e., it was found that the average density of magnetotactic bacteria is 1.19 g/cm³ from their data. The experiment was done with 5 keV X-ray photons at BL29XU, SPring-8, Japan. For more information, see the paper, "Quantitative Imaging of Single Unstained Magnetotactic Bacteria by Coherent X.ray Diffraction Microscopy", Jiadong Fan et al., Anal. Chem. 87, 5849 (2015).

Professors D. A. Keen (Rutherford Appleton Laboratory) and A. L. Goodwin (University of Oxford) have recently published an interesting review paper on disordered structures. For many years, crystallographers have determined the structures of many complicated crystals with atomic or even sub-atomic resolution. On the other hand, the structures of disordered systems, which lack the crystalline periodic order, are still not well understood because of the limits of the analytical technique. Correlated disorder is a disorder, but maintains crystallographic signatures, which can be used for classifying the type of disorder. For more information, see the paper, "The crystallography of correlated disorder", D. A. Keen and A. L. Goodwin, Nature, 521, 303 (2015).

Most X-ray experiments can be done at high quality with ease in an ordinary laboratory. Some experiments, however, have to be done in the field. It is hard to imagine a more extreme definition of "in the field" than the planet of Mars, which is why exciting times have come about since NASA's Mars rover "Curiosity" landed on Mars in August 2012. It has since recorded and sent back a large number of datasets including X-ray fluorescence (XRF) and X-ray diffraction (XRD) data. Naturally, the scientists involved with the projects have been speaking globally since. During EXRS 2014 (June, Bologna, Italy), Professor J. L. Campbell (University of Guelph, Canada) gave a keynote lecture entitled "XRF and PIXE on the Mars Science LAB Curiosity Rover". At the Denver X-ray conference (July, Big Sky, Montana), the Plenary Session was "X-rays on Mars", and 3 scientists gave lectures. Professor D. L. Bish (Indiana University) gave a talk entitled "The First X-ray Diffraction Results From Mars". Professor J. L. Campbell's talk on "XRF Combines with PIXE in Curiosity's Alpha Particle X-ray Spectrometer" was the extension on his talk at EXRS 2014, and further detailed and specific discussion was done there. Professor S.M. Clegg talked about "Exploring Mars with ChemCam on the Curiosity Rover" (ChemCam enables quick element determination by the laser-induced plasma emission spectroscopy). In August, at Montreal, during the International Union of Crystallography's congress, Professor D. L. Bish gave a talk entitled "The First X-ray Powder Diffraction Measurements on Mars". These talks highlighted many interesting technological aspects of the measurements: XRF analysis is done first by the same CCD camera, which works as an energy-dispersive 2D X-ray detector, even when the main aim of the measurement is obtaining the XRD pattern. In the analysis of unknown samples, generally both chemical composition and the crystal structure are indispensable. Another reason is that XRF helps the systematic use of single photon counting mode of the CCD camera to get a good quality XRD pattern. Secondly, the samples are vibrated all the time to ensure a smooth and continuous Debye ring. The rover furthermore contains a series of standard samples to check the reliability and reproducibility of the measurements. The readers might be interested in such a compact X-ray analyzer, which combined both XRD and XRF machine. Very similar system is now commercially available. For further information on the scientific activity on Mars, visit the Web page, http://mars.jpl.nasa.gov/msl/

Coherent X-ray diffraction imaging is a promising new technique to observe samples in material science and biology with a spatial resolution of around 10 nm. However, the range of applications is still not very wide, because the method requires that the X-ray source be highly coherent both laterally and longitudinally. Thus, one of the most important questions for users is the feasibility of the technique when only a partially coherent source is available. A research group led by Professor K. Nugent (University of Melbourne, Australia) has recently reported some quite good news on this issue. So far, it has been often said that the lateral coherence length should be at least twice the greatest spatial extent of the object. The longitudinal coherence length is determined by the bandwidth of the monochromatic X-ray beam. According to the present study, one could relax the minimal criteria by a factor of 2 for both lateral coherence length and longitudinal coherence length, if the coherence properties are known either a priori or through experiment. In other words, more flux could be made available at the sample position for the coherent X-ray diffraction imaging experiments with the use of a partially coherent X-ray source. For more information, see the paper, "Diffraction imaging: The limits of partial coherence", B. Chen et al., Phys. Rev. B86, 235401 (2012).

A German group at Karlsruhe Institute of Technology has recently reported a quick X-ray diffraction experiment during laser surface hardening of materials. They employed a single exposure setup with two fast silicon strip line detectors (Mythen 1K, Dectris Ltd.), allowing for stress analysis according to the sin²ψ profile, and the measurements were done at beamline P05, PETRA III, DESY, Hamburg in Germany. A 6 kW diode laser was used for hardening of the material at a heating/cooling rate of 1000 K/s. In the paper, they described how they can perform high-resolution strain analysis by separating elastic and thermal strains. For more information, see the paper, "Fast in situ phase and stress analysis during laser surface treatment: A synchrotron x-ray diffraction approach", V. Kostov et al., Rev. Sci. Instrum., 83, 115101 (2012).

One of the remarkable instances of progress in soft-X-ray spectroscopy recently is the successful high-resolution measurement of O-K edge absorption spectra of liquid water and ice, which have some disordered hydrogen-bonds. Professor R. Car (Princeton University) and his colleagues have recently reported their theoretical studies into the quantum dynamics of the nuclei and inhomogeneous screening effects. They found that the inclusion of quantum disorder is essential to bring the calculated spectra in close agreement with the experiment. In particular, the intensity of the pre-edge feature, a spectral signature of broken and distorted hydrogen bonds, is accurately reproduced, in water and hexagonal ice, only when quantum nuclei are considered. The effect of the inhomogeneous screening is less important but non-negligible, particularly in ice. For more information, see the paper, "Roles of quantum nuclei and inhomogeneous screening in the x-ray absorption spectra of water and ice", L. Kong et al., Phys. Rev. B86, 134203 (2012).

Extremely strong pulses from X-ray free electron laser (XFEL) can change the material structure. Recently, scientists at LCLS (Linac Coherent Light Source), Stanford, USA, have reported the amorphous to crystalline phase transition of carbon by femtosecond 830 eV XFEL beam. The research group employed atomic force microscopy, photoelectron microscopy, and micro-Raman spectroscopy to discuss the change of the sp²/sp³ ratio (graphitization), as well as the change of local order of the irradiated sample area. It was found that the phase transition threshold fluence is 282 ± 11 mJ/cm², and also the transition is mainly due to thermal activation rather than a non-thermal mechanism such as ionization etc. For more information, see the paper, "Amorphous to crystalline phase transition in carbon induced by intense femtosecond x-ray free-electron laser pulses", J. Gaudin et al., Phys. Rev. B86, 024103 (2012).

A research team led by Professor A. Adriaens (Ghent University, Belgium) has developed a number of useful techniques based on synchrotron X-ray diffraction to see the growth of synthetic corrosion layers in real time. The observation was done for copper, and the final products were identified as mixtures of nantokite (CuCl), cuprite (Cu₂O), and paratacamite (Cu₂(OH)₃Cl). The team employed a highly sophisticated instrument for growing corrosion using a spin coater, and it could be used for many other similar applications. Experiments were done at both SRS, Daresbury and ESRF, Grenoble. For more information, see the paper, "The Use of Synchrotron X-rays To Observe Copper Corrosion in Real Time ", M. Dowsett et al., Anal. Chem. 84, 4866 (2012).

An interesting paper has been published showing the extended X-ray absorption fine structure (EXAFS) as evidence of negative expansion of CdTe crystal. Measurements were done for both the K edges of cadmium and tellurium, from 4.2 K to room temperature. For more information, see the papers, "Negative thermal expansion in crystals with the zincblende structure: an EXAFS study of CdTe", N Abd el All et al., J. Phys.: Condens. Matter 24, 115403 (2012).

Multi-wavelength anomalous diffraction (MAD) has been widely employed to determine phase information in X-ray crystallography. The method uses the contrast of the scattering power of heavy atoms at the absorption edges. However, when the X-ray source becomes extremely brilliant, the sample encounters severe electronic radiation damage, especially to heavy atoms, which makes the interpretation of MAD rather difficult. Recently, a theoretical paper discussing this problem has been published. The theory uses a Karle-Hendrickson-type equation in the high-intensity regime, and demonstrates the calculation of relevant coefficients with detailed electronic damage dynamics of heavy atoms. For more information, see the paper, "Multiwavelength Anomalous Diffraction at High X-Ray Intensity", S-K.Son et al., Phys. Rev. Lett. 107, 218102 (2011).

One of the hottest topics in X-ray structural analysis is coherent X-ray diffraction measurement to obtain real-space images of nanoscale crystals. The key here is the method of phase retrieval. Until now, iterative projective algorithms have been frequently employed to recover the phase information from the amplitude measured in reciprocal space. The analysis relies on experimental data to be oversampled, and there have been difficulties in the case of highly strained structures, where information is below the Nyquist frequency. A research team led by Professor I. K. Robinson (University College London, UK) has recently reported a new method, called a density modification phase reconstruction algorithm, to solve this problem. This is a successful extension of the recent compressive sensing theory and works well in solving the nonconvex phase retrieval problem for highly strained crystalline materials. For more information, see the paper, "Phase retrieval of diffraction from highly strained crystals", M. C. Newton et al., Phys. Rev. B 82, 165436 (2010).

A research team led by Professor I. Robinson (London Centre for Nanotechnology, University College London) recently analyzed how gold nanocrystal changes after the adsorption of organic molecules because of the strain field. So far, it has been difficult to observe such influence of adsorbed molecules on the particle structure. The team employed the coherent X-ray diffraction method, which is extremely sensitive to displacement of atoms, and therefore to adsorption-induced near-surface stress in a single nanocrystal. It was discovered that the stress generated by thiol adsorption on gold has a fundamentally different nature in the curved, nominally spherical, regions of the crystal surface than in its flat facets. The magnitude of surface stress was also quantitatively analyzed and discussed. The experiments were done with coherent X-rays of 8.92 keV from the 34-ID-C beamline of the Advanced Photon Source (APS), Argonne, USA. For more information, see the paper, "Differential stress induced by thiol adsorption on facetted nanocrystals", M. Watari et al., Nature Materials 10, 862 (2011).

A German group led by Professor U. Klemradt (Aachen University) has recently performed an X-ray photon correlation spectroscopy (XPCS) experiment on martensitic transformation of a Au_50.5Cd_49.5 single crystal. XPCS experiments basically consist of the observation of a time-dependent speckle pattern caused by scattering of coherent X-ray photons, and give information on the dynamics of phase transformations in soft and hard condensed matter at atomic length scales. The measurement was done at ID 10A, European Synchrotron Radiation Facility (ESRF) in Grenoble, France. A standard Bragg scattering geometry was employed to see the fluctuations of the symmetric (0 0 1) Bragg reflection from the polished surface of the Au-Cd single crystal. The research team observed slow non-equilibrium-dynamics in a narrow temperature interval in the direct vicinity of the otherwise athermal phase transformation. For more information, see the paper, "Slow Aging Dynamics and Avalanches in a Gold-Cadmium Alloy Investigated by X-Ray Photon Correlation Spectroscopy", L. Muller et al., Phys. Rev. Lett. 107, 105701 (2011).

Professor M. D. Ward (New York University, USA) and his colleagues have recently proposed an interesting and effective application of the micro X-ray diffraction technique to anticounterfeit protection of pharmaceutical products. Counterfeit drugs have been a global threat to public health, and they undermine the credibility and the financial success of the producers of genuine products. There have been great demands for some good methods for rapid and nondestructive screening of the products. The research team's idea is the use of barcodes and logos fabricated on drug tablets using soft-lithography stamping of compounds that can be read by X-ray diffraction mapping but are invisible to the naked eye or optical microscopy. The materials used were suspensions of rutile powder mixed with corn syrup in a 1:2.5 (w/w) ratio or zinc oxide powder mixed with corn syrup at a 1:10 (w/w) ratio. It was demonstrated that the technique is feasible for realistic screening, because of its nondestructive, automated, and user-friendly properties. For more information, see the paper, "Anticounterfeit Protection of Pharmaceutical Products with Spatial Mapping of X-ray-Detectable Barcodes and Logos", D. Musumeci et al., Anal. Chem., Articles ASAP (DOI: 10.1021/ac201570r Publication Date (Web): August 30, 2011).

The research team led by Professors V. Holý (Charles University, Czech Republic) and T. Baumbach (ANKA-Institute for Synchrotron radiation, Germany) have recently performed some extension of coherent X-ray diffractive imaging for high-resolution strain analysis in crystalline nanostructured devices such as layered nanowires and/or dots. Their research successfully determined the strain distribution in (Ga,Mn)As/GaAs nanowires. The key was their improvement of the phase-retrieval algorithm, i.e., separation of diffraction signals in reciprocal spaces. It was found that individual parts of the device can be reconstructed independently by this inversion procedure. The method is effective even for strongly inhomogeneously strained objects. For more information, see the paper, "Selective coherent x-ray diffractive imaging of displacement fields in (Ga,Mn)As/GaAs periodic wires", A. A. Minkevich et al., Phys. Rev. B84, 054113 (2011).

Professor K. F. Ludwig (Boston University, USA) and his colleagues have recently reported their real-time X-ray scattering studies on heterogeneous microscale dynamics in the martensitic phase transition of cobalt. During the transformation of the high-temperature fcc phase to the low-temperature hcp phase, first, a rapid local transformation happens, and then, strains are relaxed slowly. The research group employed coherent X-ray scattering measurements to see the latter part of the transformation. It was found that the kinetics is dominated by discontinuous sudden changes - avalanches. The spatial size of observed avalanches varies widely, from 100 nm to 10μm, the size of the X-ray beam. For more information, see the paper, "Direct Measurement of Microstructural Avalanches during the Martensitic Transition of Cobalt Using Coherent X-Ray Scattering", C. Sanborn et al., Phys. Rev. Lett. 107, 015702 (2011).

A research group led by Professor L. Vincze (Ghent University, Belgium) has recently reported the interesting analysis of 1-20 μm sized inclusions in natural diamond crystals from Rio Soriso (Juina area, Mato Grosso State, Brazil). The crystals are called ultra-deep diamond, because they were formed in the astenospheric upper mantle, the transition zone (410-670 km), and even the lower mantle (>670 km) of the Earth. The experiment is basically 3D imaging by confocal X-ray fluorescence suing synchrotron radiation. By scanning X-ray energy near the Mn and Fe K absorption edges, the authors obtained chemical information on the inclusion cloud in the crystal. It was found that the observed Fe-rich inclusions were ferropericlase (Fe,Mg)O, hematite and a mixture of these two minerals. Another finding was that significant overprint of inclusions along pre-existing planar features is possible without changing their outer shape. For more information, see the paper, "Three-Dimensional Fe Speciation of an Inclusion Cloud within an Ultradeep Diamond by Confocal μ-X-ray Absorption Near Edge Structure: Evidence for Late Stage Overprint", G. Silversmit et al., Anal. Chem., Article ASAP (DOI: 10.1021/ac201073s Publication Date (Web): June 27, 2011).

Ptychographic X-ray diffraction microscopy is known as an extension of so-called X-ray diffraction microscopy, which is a lensless X-ray imaging technique based on coherent diffraction measurements and iterative phasing methods. The technique employs sample scanning to see a large viewing area, but so far, the spatial resolution has been rather limited mainly because of positioning errors due to the drift between the sample and illumination optics. Recently, Professor Y. Takahashi (Osaka University, Japan) and his colleagues have published an experimental way to resolve the problem. The research group has developed a method of correcting positioning errors, and made it possible to illuminate a highly focused hard X-ray beam at the exact position on the samples. The spatial resolution achieved is as good as 10 nm or even better in a viewing area of larger than 5 μm. For more information, see the paper, "Towards high-resolution ptychographic X-ray diffraction microscopy", Y. Takahashi et al., Phys. Rev. B83, 214109 (2011).

Inelastic X-ray scattering is a powerful modern tool to study lattice dynamics of condensed matter. Recently an international team led by Dr. J. Serrano (Polytechnic University of Catalonia, Spain) has tried to extend the technique to several micron-thick systems by introducing grazing-incidence geometry. Their sample is indium nitride grown on a sapphire substrate with a gallium nitride buffer layer inbetween, but X-rays only probe the surface, and not the substrate underneath. The analysis was combined with ab initio calculations to determine the complete elastic stiffness tensor, the acoustic and low-energy optic phonon dispersion relations. This finding could be a help in developing new types of solar cells. For more information, see the paper, "InN Thin Film Lattice Dynamics by Grazing Incidence Inelastic X-Ray Scattering", J. Serrano et al., Phys. Rev. Lett. 106, 205501 (2011).

A German group led by Professor U. Panne (Humboldt University, Berlin) has recently reported the successful application of the micro X-ray diffraction technique to the evaluation of the durability of cements against reaction with sodium sulfate. The experiments were done with a Debye-Scherrer camera equipped with a large-size CCD camera (3072×3072) and monochromatic micro beam (11.6 keV, 10 μm). By moving the sample along the X-ray path, it is possible to obtain information at different depths, and the team could therefore eventually reconstruct the profile of each crystalline phase along the depth from the surface. It was found that phase transformations proceeded during damage caused by penetration of sulfates. For more information, see the paper, "Deciphering the Sulfate Attack of Cementitious Materials by High-Resolution Micro-X-ray Diffraction", M. C. Schlegel et al., Anal. Chem., 83, 3744 (2011).

One of the hottest topics in X-ray crystallography in the early 21^st century is coherent X-ray diffraction imaging and its application to the determination of atomic structures of non-crystalline materials - the ultimate goal can be a single molecule. The technique appears to require non-ordinary coherent photon sources, such as X-ray free-electron lasers (XFEL), which are now in operation at Stanford. On the other hand, there are several challenging questions basically concerning sample damage, Coulomb explosion, and the role of nonlinearity. Recently, Dr. A. Fratalocchi and his colleague published their calculations showing that XFEL-based single-molecule imaging will only be possible with a few-hundred long attosecond pulses, due to significant radiation damage and the formation of preferred multisoliton clusters which reshape the overall electronic density of the molecular system at the femtosecond scale. For more information, see the papers, "Single-Molecule Imaging with X-Ray Free-Electron Lasers: Dream or Reality?", A. Fratalocchi et al., Phys. Rev. Lett. 106, 105504 (2011).

Two very exiting experimental reports have been published on the application of an X-ray free electron laser (XFEL) at Linac Coherent Light Source (LCLS, Stanford, USA). An international research team led by Dr. H. Chapman (DESY, Hamburg, Germany) and Professor J. Hajdu (Uppsala University, Sweden) has demonstrated a new advanced stage of protein crystallography, which uses only tiny proteins instead of preparing large-size crystals. This could open up new possibilities for the analysis of proteins that have been difficult or even impossible to prepare so far. The technique has been basically known as coherent X-ray diffraction imaging. The present research is the first experimental application of extremely brilliant femtosecond XFEL pulses. In addition to the demonstration of snapshots of nano-crystalline proteins, they have reported the first single-shot images of intact viruses. For more information, see the papers, "Femtosecond X-ray protein nanocrystallography", H. N. Chapman et al., Nature, 470, 73 (2011) and "Single mimivirus particles intercepted and imaged with an X-ray laser", M. M. Seibert et al., Nature, 470, 78 (2011).

Scientists in Japan have been using two synchrotrons, the SPring-8 and the Photon Factory, to analyze the dust particles collected by the HAYABUSA Asteroid probe, which returned from Asteroid Itokawa on June 13, 2010. HAYABUSA, which means "Falcon" in Japanese, was launched from the Uchinoura Space Center in Japan on May 9, 2003, and arrived at Itokawa in September 2005. The HAYABUSA particles were initially analyzed using electron microscopes, and then forwarded to the above synchrotron facilities in January 2011. Many interesting 3D images were collected at BL20XU, SPring-8, and the structure and chemical compositions were also analyzed at BL-3A, Photon Factory, KEK. For more information on the HAYABUSA project, visit the web page of the Japan Aerospace Exploration Agency (JAXA), http://www.isas.jaxa.jp/e/enterp/missions/hayabusa/index.shtml

One of hottest topics related to the application of an X-ray free electron laser (XFEL) is how to determine the structure of non-crystalline membrane proteins. There has been a clear conflict between the incident brightness required to achieve diffraction-limited atomic resolution and the electronic and structural damage induced by such illumination. Professors K. A. Nugent and H. M. Quiney (ARC Centre of Excellence for Coherent X-ray Science, University of Melbourne, Victoria, Australia) have recently published their theoretical research on this problem. They have improved the imaging model by using optical coherence theory and quantum electrodynamics, and concluded that the analysis is far more tolerant of electronic damage than believed so far. For more information, see the paper, "Biomolecular imaging and electronic damage using X-ray free-electron lasers", H. M. Quiney et al., Nature Physics, 7, 142 (2011).

Professor M. P. Fontana (University of Parma, Italy) and his colleagues have recently reported X-ray photon correlation spectroscopy (XPCS) studies on poly[[4-pentiloxy-3'-methyl-4'-(6-acryloxyexyloxy)]azobenzene], which is a kind of photosensitive azo-polymer and is softened by photoisomerization. XPCS uses coherent X-rays to measure small angle scattering, called a speckle pattern, which is caused by some inhomogeneities. It gives information on the slow dynamics of various equilibrium and non-equilibrium processes in condensed matter systems. The main advantage of using X-rays instead of other direct methods such as scanning probe microscopy is that it provides statistical information averaged over the whole sample as a function of the momentum transfer. This is essential for the analysis of dynamical heterogeneity and of nonequilibrium and aging effects in the observed dynamics. The research group measured the time correlation functions at different temperatures and momentum transfers (q) and under different illumination conditions (dark, UV or blue light). It was found that the correlation functions are well described by the so-called stretched exponential function with relaxation times that are proportional to the inverse of q. They were able to determine the scaling laws for equilibrium and nonequilibrium fluctuations on local space scales. For more information, see the paper, "Slow dynamics in an azopolymer molecular layer studied by x-ray photon correlation spectroscopy", D. Orsi et al., Phys. Rev. E82, 031804 (2010).

Some readers might remember the news article, "A new technique with coherent X-rays to determine non-crystalline structures", in X-ray Spectrometry, Vol. 38, No.5 (2009). The technique called X-ray cross correlation analysis (XCCA) is an extension of X-ray photon correlation spectroscopy, and is promising with respect to solving the atomic-scale structures of complicated disordered systems, which have for many years presented difficulties in terms of reaching a clear understanding of the structures. Recently, Dr. M. Altarelli (European X-ray Free-Electron Laser Facility, Hamburg, Germany) and his colleagues published a paper on the theoretical treatment of XCCA. They gave a general theory for the cross correlation function, and tried to interpret the experimental XCCA results for colloidal glass. The authors plan further publications to present the results of various simulations as well. For more information, see the paper, "X-ray cross-correlation analysis and local symmetries of disordered systems: General theory", M. Altarelli et al., Phys. Rev. B82, 104207 (2010).

A geoscientists group at the European Synchrotron Radiation Facility (ESRF, Grenoble, France), has recently found that a natural fertile peridotite, which is a characteristic material of the Earth's mantle, can be partially molten at a pressure of 140 GPa, when the temperature reaches 4,200 K. This could reinforce the hypothesis of the presence of a deep magma ocean. The experiments showed that the liquid produced during this partial fusion is dense and that it can hold multiple chemical elements, among which are important markers of the dynamics of the mantle. For more information, see the paper, "Melting of Peridotite to 140 Gigapascals", G. Fiquet et al., Science, 329, 1516 (2010).

Soft X-ray resonant diffraction and reflectivity have become one of the most promising tools with which to study magnetic materials. At Diamond Light Source, Oxfordshire, UK, a novel instrument for single crystal diffraction and thin film reflectivity experiments in the soft X-ray regime has been designed and constructed. It is basically a limited three circle (q, 2q, and c) diffractometer with an additional removable rotation (f), and is equipped with a liquid helium cryostat, and post-scatter polarization analysis. For more information, see the paper, "RASOR: An advanced instrument for soft x-ray reflectivity and diffraction", T. A. W. Beale et al., Rev. Sci. Instrum. 81, 073904 (2010).

Combinatorial materials synthesis is a promising new way of developing and finding novel functional materials. By the use of sophisticated thin film technology, it is possible to create compositionally graded samples on the same single substrate. To analyze this combinatorial library, some novel technique is required. A UK research group led by Professor K. D. Rogers (Cranfield University, UK) recently reported on high-throughput data collection and analysis using an X-ray diffraction (XRD) probe. In the research, an extended X-ray beam was used to illuminate the libraries, and a large area detector was used to collect the data. A new algorithm was employed to analyze the collected data and extract the crystallographic information. For more information, see the paper, "High Throughput X-ray Diffraction Analysis of Combinatorial Polycrystalline Thin Film Libraries", S. Roncallo et al., Anal. Chem., 82, 4564 (2010).

Coherent X-ray diffraction imaging is one of the hottest research topics in advanced X-ray physics. The method reconstructs a real-space image from an oversampled diffraction signal by using computer algorithms instead of lenses. So far, its application has been limited to fairly strong phase objects, mainly due to parasitic scattering from the optics used for limiting the beam. Korean researchers recently published an interesting report on its application to a nonisolated weak phase object, a one-dimensional trench structure fabricated on a Si substrate. In their discussion, the authors reported that such work was enabled by employing a special aperture with a very high aspect ratio of nearly 100 made of tantalum (1.7 μm × 2.2 μm aperture with a thickness of 130 μm). For more information, see the paper, "Coherent hard x-ray diffractive imaging of nonisolated objects confined by an aperture", S. Kim et al., Phys. Rev. B81, 165437 (2010).

There are still many unknown problems related to the structure of amorphous materials, because the X-ray diffraction technique has some limitations in the case of disordered systems. A research team led by Dr. A. L. Goodwin (Oxford University, UK) recently reported a new elegant general scheme to solve the structure by successfully demonstrating its application to molecular C₆₀, a-Si, and a-SiO₂. The team proposes to employ the information gained in spectroscopic experiments (such as EXAFS, Raman, NMR etc) regarding the number and distribution of atomic environments. The idea is that such information can be used as a valuable constraint in the refinement of the atomic-scale structures of nanostructured or amorphous materials from the pair distribution function (PDF), which is obtained by Fourier transform of the X-ray diffraction pattern. Although a conventional reverse Monte Carlo (RMC) approach is not always successful in obtaining the correct structure solution, the team showed that such difficulties can be removed by including the above variance term. For more information, see the paper, "Structure Determination of Disordered Materials from Diffraction Data", M. J. Cliffe et al., Phys. Rev. Lett. 104, 125501 (2010).

Professor S. Techert (Max-Planck-Institute for Biophysical Chemistry, Goettingen, Germany) and his colleagues have reported on Bragg diffraction experiments with a soft X-ray laser (wavelength 8 nm, pulse width 30 fs, power 4×10¹¹ photons/pulse) from the free electron laser at FLASH, Deutsche Elektronen-Synchrotron (DESY) in Hamburg. The research group studied Bragg diffraction patterns of single nano-crystal (20 nm×20 nm×20 μm) and powder with grain sizes smaller than 200 nm of silver behenate (AgC₂₂H₄₃O₂, chain length 5.8 nm). So far, many coherent X-ray diffraction studies have been done even with soft X-ray wavelengths, but the present research aims at the analysis of periodic structures that are usually targets of X-ray diffraction with hard X-rays. They showed an interesting comparison between the single nano crystal and the powder, and also discussed the influence of the extremely high peak power of laser pulses. For more information, see the paper, "Diffraction Properties of Periodic Lattices under Free Electron Laser Radiation", I. Rajkovic et al., Phys. Rev. Lett. 104, 125503 (2010)

A research group led by Professors Y. Takanishi (Kyoto University, Japan) and A. Iida (Photon Factory, KEK, Japan) has recently published its successful investigation into the local layer structure of bent-core liquid crystal, 4-Br-14-O-PIMB, which includes Br atoms. The group employed a monochromatic X-ray microbeam (3 μm × 4 μm), and observed X-ray scattering from the cell near the Br K absorption edge. They were able to discover some satellite peaks reflecting the superlattices. For more information, see the paper, "Microbeam resonant x-ray scattering from bromine-substituted bent-core liquid crystals", Y. Takanishi et al., Phys. Rev. E81, 011701 (2010).

X-ray Photon Correlation Spectroscopy (XPCS) is a novel technique which reveals the slow dynamics of equilibrium and non-equilibrium processes in condensed matter systems. A group led by Professor N. P. Balsara (University of California, Berkeley, USA) has recently published research on a polystyrene-polyisoprene block copolymer melt in the vicinity of the order-disorder transition. The group combined several techniques in addition to XPCS; time-resolved small angle X-ray scattering and rheology. During their studies of ordering kinetics, it was found that two qualitatively different regimes exist, i.e., shallow and deep quench regimes, respectively. For more information, see the paper, "Dynamic signatures of microphase separation in a block copolymer melt determined by X-ray photon correlation spectroscopy and rheology", A. J. Patel et al., Macromolecules, Article ASAP (DOI: 10.1021/ma902343m).

Nanometer scale dipole moments in the polarization clusters in BaTiO₃ are believed to be thermally excited and thermally relaxed within a picosecond time scale. However, so far, there have been no reports on the direct observation of the dynamics of these dipole moments in such a very short time scale. The limitation here is mainly due to the low spatial coherence of the X-ray beam, in particular when synchrotron radiation is used as a light source. Professor K. Namikawa (Tokyo Gakugei Univ, Japan) and his colleagues have recently obtained some interesting results. To measure the time correlation of speckle intensities, they employed a soft X-ray pulse laser (7 ps in pulse width, 3.5×10¹⁰ photons/sec/pulse, 13.9 nm in wavelength, band width 10^-4, angular spread 0.5 mrad) at Japan Atomic Energy Agency, Kizugawa, Japan, and a Michelson-type delay pulse generator as well as an X-ray streak camera. Spatial coherence in their system was estimated at more than 90 %. The evolution of the relaxation time of the dipole moment near the Curie temperature (TC) was studied. It was found that the maximum relaxation time (~90 ps) appears at a temperature of 4.5 K above the TC, being coincident with the one where the maximum polarization takes place. For more information, see the paper, "Direct observation of the critical relaxation of polarization clusters in BaTiO₃ using a pulsed X-ray laser technique", K. Namikawa et al., Phys. Rev. Lett., 103, 197401 (2009).

So far, X-ray microscopy with many types of lens has achieved great success in the observation of biological cells. In order to extend the limits of spatial resolution and efficiency, X-ray diffraction microscopy (also called coherent X-ray diffraction imaging), which uses coherent X-rays and some image reconstruction algorithms instead of an optical lens system, is now considered as a promising procedure to see whole cells at once and pick out much smaller features, down to around 10 nm or even less. A research group led by Professor C. Jacobsen (Stony Brook University, USA) recently reported the results for yeast cells with 520 eV soft X-rays at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, USA. Dr. A. Madsen (European Synchrotron Radiation Facility (ESRF), Grenoble, France) and his colleagues observed the cells of the bacteria D. radioduran with 8 keV X-rays. The advantage of using hard X-rays is the ease of sample handling, and the validity of thin sample approximation for future 3D reconstructions through phasing a diffraction volume. In both cases, a rapid freezing technique (instead of previously used freeze-drying) was used to avoid the effects of radiation damage from synchrotron X-ray photons. The Stony Brook group plunged cells in their natural wet state into liquid ethane and maintained them at below -170 ^oC, leading to the reduction of artifacts due to damage from dehydration, ice crystallization, and radiation. In the ESRF setup, as absorption in air of 8 keV X rays is small, a nonvacuum environment was implemented for ease of sample handling. Similar to the system for macromolecular crystallography applications, they based the samples in a continuous cryogenic nitrogen gas jet at around -165 ^oC. The spatial resolution was 25 nm and 30-50 nm, for soft and hard X-rays cases, respectively. For more information, see the papers, "Soft X-ray diffraction microscopy of a frozen hydrated yeast cell", X. Huang et al., Phys. Rev. Lett., 103, 198101 (2009), and "Cryogenic X-ray diffraction microscopy for biological samples", E. Lima et al., Phys. Rev. Lett., 103, 198102 (2009)

So far, diffusion in solids has been investigated by profiling the depth dependence of tracer atoms diffused into the sample. Although one can obtain the diffusion constant from this, the question is how diffusion takes place on the atomic scale, rather than on the micron scale. Sometimes quasielastic neutron scattering as well as Mobauer spectroscopy can be used in a very limited number of fortunate cases. A research group led by Professor G. Vogl (University of Vienna, Austria) recently reported the use of X-ray photon correlation spectroscopy (XPCS) to observe the dynamics of diffusing atoms. The research was done for intermetallic alloy Cu₉₀Au₁₀, at temperatures of around 540 K, where the system is a substitutional solid solution, that is, the Au atoms statistically occupy sites in the Cu fcc lattice. The research gives the dynamical behavior of single atoms as a function of their neighborhood, and confirms quantitatively that Au atoms have a tendency to locally order on a certain set of sites in the crystal. Photon correlation spectroscopy is based on analysis of 'speckle' patterns, which are fine-scale diffraction patterns that appear in the scattering of coherent light from a disordered system. Speckle patterns are sensitive to the exact spatial arrangement of the disorder. By observing the intensity fluctuations in the speckle pattern, the characteristic times of fluctuations in the system can be determined. For more information, see the paper, "Atomic diffusion studied with coherent X-rays", M. Leitner et al., Nature Materials,8, 717 (2009).

In X-ray diffraction experiments, one measures the intensity (amplitude) of the diffracted X-rays as a function of position in the reciprocal space, and the information on the phase is always missing. For many years, this so-called phase problem has been thought as one of the biggest problems in X-ray crystallography. Professor E. Wolf (University of Rochester, New York) has recently published a very interesting and inspirational paper. He is famous for several important textbooks on optics and also for his presidency of the Optical Society of America. The present paper is theoretical, and starts with a criticism of basic understanding of the problem. The author says that trying to measure the phase is rather meaningless. Almost all scientists assume that the incident X-ray beam is monochromatic in the data analysis, but the author points out that a monochromatic beam is not possible in reality. Any beam that can be produced in a laboratory is, at best, quasimonochromatic and, therefore, even if both the amplitudes and the phases are given, it is still not possible to solve the problem. Alternatively, the author proposes the measurement of certain correlation functions, with the use of spatially coherent beams. While it is extremely important to think about a future strategy regarding the final solution of the phase problem as discussed in the paper, the author makes no mention of the recent significant strides in coherent X-ray scattering. For more information, see the paper, "Solution of the Phase Problem in the Theory of Structure Determination of Crystals from X-Ray Diffraction Experimentst", E. Wolf, Phys. Rev. Lett. 103, 075501 (2009).

Professor H. Dosch (Director of Deutsches Elektronen-Synchrotron (DESY), Germany) and his colleagues recently published a very interesting paper on the symmetry of disordered systems. They propose a new technique, X-ray cross correlation analysis (XCCA). This measures X-ray speckles and is basically an extension of X-ray photon correlation spectroscopy (XPCS). The samples studied were colloidal glasses, and the research group was able to observe clear symmetries that conventional X-ray diffraction has been unable to extract. The research group recommends using brilliant coherent X-ray sources, such as X-ray free electron lasers for future research. For more information, see the paper, "X-ray cross correlation analysis uncovers hidden local symmetries in disordered matter", P. Wochnera et al., Proc Nat Aca Sci, 106, 11511 (2009).

The use of short pulses of extremely bright synchrotron X-rays has opened up a new world. In Japan, Dr. S. Adachi (KEK, Tsukuba Japan) and his colleagues recently succeeded in recording movies during changes in the molecule structures of myoglobin. The samples used are frozen myoglobin crystals that had CO (carbon monoxide) stored inside before the start of the experiments. Even at 100K, irradiating pulsed laser light gave the trigger for the migration of CO molecules. To see changes in atomic scale, time-resolved X-ray diffraction measurements were performed. The obtained movie tells us that the CO molecules penetrate into a number of cavities in the crystal and even expand their size. The research group has obtained an important result suggesting some self-opening mechanism in the ligand migration channel. For more information, see the paper, "Visualizing breathing motion of internal cavities in concert with ligand migration in myoglobin", A. Tomita et al., Proceedings of National Academy of Science, 106, 2612-2616 (2009) Published online before print February 9, 2009, doi: 10.1073/pnas.0807774106

In classical metallurgy, there exists a very famous rule known as Hume-Rothery's rule, which describes the conditions necessary for the formation of a solid solution from two independent metals. In order to have a substitutional crystalline solid solution in which the atoms of one element randomly substitute for atoms of another element in a crystal structure, the components must have an atomic size within 15% and electronegativity within 0.4 of each other. According to this rule, a Ce-Al solid solution cannot be obtained. Recently, a research team led by Professor H.K. Mao (Carnegie Institution of Washington) and Professor R. Ahuja (Uppsala University) found during high pressure research on the intermetallic compound of Ce₃Al that a solid solution is formed in a Ce-Al system. The differences in radii and electronegativity of Ce and Al were diminished by applying pressure. Both synchrotron X-ray studies (XRD and X-ray absorption spectroscopy) and ab initio calculations showed the same cause for bringing the two elements closer in radii and electronegativity, resulting in the new alloy phase. Even after the release of pressure, this substitutional alloy remained. During in-situ X-ray absorption measurements at the Ce LIII edge, conspicuous changes in the sharpness of the absorption, correlated to delocalization of 4f electrons, were observed. For more information, see the paper, "Substitutional alloy of Ce and Al", Q-S.Zeng et al., Proceedings of National Academy of Science, 106, 2515-2518 (2009) Published online before print February 2, 2009, doi: 10.1073/pnas.0813328106

Diffractive imaging is a technique for so-called lens-less microscopy, and uses diffraction intensity (image) and phase retrieval calculations rather than focusing systems such as lenses, which are not free from aberrations. The spatial resolution is basically limited only by the amount of high-angle scattering. Therefore, the technique has been considered as having the potential to achieve atomic resolution for hard X-rays or other short-wavelength particle beams. However, so far, the reported results have been still at the level of several nanometers. Recently, a research group at the University of Illinois, USA proposed a method of improving the resolution. One of the biggest technical reasons limiting the spatial resolution of diffractive imaging is the difficulty of recording weak coherent scattering signals. The research group proposes the combined use of low-resolution imaging, which provides the starting phase, real-space constraint, missing information in the central beam and essential marks for aligning the diffraction pattern. The group used an electron microscope to see a single CdS quantum dot with sub-angstrom resolution and noted that it is possible to use the same procedure in the case of coherent X-ray scattering. For more information, see the paper, "Sub-angstrom-resolution diffractive imaging of single nanocrystals", W. J. Huang et al., Nature Physics, advanced online publication doi:10.1038/nphys1161

Professor A. Cupane (University of Palermo, Italy) and his colleagues at the European Synchrotron Radiation Facility (ESRF) recently established a method for structural dynamics. The technique uses wide-angle X-ray scattering and images proteins in their natural, fast-moving state. The research group succeeded in capturing the tertiary and quaternary conformational changes of human hemoglobin in close to physiological conditions triggered by laser-induced ligand photolysis. The time resolution of the observation is in the order of nsec. The whole process lasts 3 μsec, and the molecule changes from a "relaxed" form that can bond to oxygen, to a "tense" form that squeezes out the oxygen. They also reported data on optically induced tertiary relaxations of myoglobin and refolding of cytochrome c. For more information, see the paper, "Tracking the structural dynamics of proteins in solution using time-resolved wide-angle X-ray scattering", M. Cammarata et al., Nature Methods, published online, 21 September 2008, doi:10.1038/nmeth.1255

It is well known that nanoparticles often enhance catalytic activity. However, it is still an open question as to whether the metallic or the oxidized state of the particle is the catalytically more active phase. It is therefore significant to study the oxidation/reduction process of metallic nanoparticles. A group led by Professor H. Dosh (Max-Planck-Institut für Metallforschung, Germany) recently reported on some very interesting XRD and GISAXS studies on the oxygen-induced shape transformation of Rh nanoparticles. The experiments were done in-situ, during the oxidation/reduction cycle at high temperature. The group found that shape transformation is driven by the formation of a surface oxide O-Rh-O trilayer, which can stabilize Rh nanoparticles with low-index facets. For more information, see the paper, "Shape Changes of Supported Rh Nanoparticles During Oxidation and Reduction Cycles", P. Nolte et al., Science, 321, 1654-1658 (2008).

Some of the most well known self-assembled monolayers (SAMs) are alkyl sulfides on gold surfaces. They have many potential applications in molecular electronics, biosensors, and nanopatterning. However, there have still been unsolved problems in basic research regarding Au-S interaction. Recently, Professor A. Morgante (Universita' di Trieste, Italy) and his colleagues published the results of grazing incidence X-ray diffraction and density functional theory-based molecular dynamics simulations for hexanethiol and methylthiol. The research group demonstrated surface complexes wherein two S atoms are joined by an intermediate Au adatom (RS-Au-SR) for longer chain cases. It was found that the sulfur atoms of the molecules bind at two distinct surface sites, and that the first surface layer contains vacancies as well as gold adatoms that are laterally bound to two sulfur atoms. Competition between SAM ordering and disordering of interfacial Au atoms takes an important role in the system. For more information, see the paper, "X-ray Diffraction and Computation Yield the Structure of Alkanethiols on Gold(111)", A. Cossaro et al., Science, 321, 943-946 (2008).

Aerogel is a form of nanofoam, an engineered material designed for its high strength-to-weight ratio for application wherever lightness and strength are needed. Now, the internal structure is within the scope of X-ray analysis. Lawrence Livermore and Lawrence Berkeley scientists have successfully applied the coherent X-ray diffraction technique to Ta₂O₅ nanofoam, the density of which is 1.2 % to the bulk, and have reconstructed 3D images to determine its strength and potential new applications. Combining the obtained structural information with detailed simulations, the research team showed that the blob-and-beam network structure explains why the materials are weaker than expected. For more information, see the paper, "Three-Dimensional Coherent X-Ray Diffraction Imaging of a Ceramic Nanofoam: Determination of Structural Deformation Mechanisms", A. Barty et al., Phys. Rev. Lett., 101, 055501 (2008).

Scanning diffraction microscopy, or ptychography, was first developed for the scanning transmission electron microscope (STEM). In the same way, by using an X-ray nano beam, one can use a STXM. The X-ray beam is focused onto the sample via a lens, and the transmission is measured. The image is obtained by plotting the transmission as a function of the sample position, as it is rastered across the beam. The analysis is straightforward, but its resolution is limited by the beam size. On the other hand, coherent diffractive imaging (CDI) now reaches resolutions below 10 nm, but the reconstruction procedures are not always easy due to the influences of data quality, sample conditions etc. A Swiss research group led by Drs. C. David and F. Pfeiffer (Paul Scherrer Institut) recently demonstrated a ptychographic imaging method that bridges the gap between STXM and CDI by measuring complete diffraction patterns at each point of a STXM scan. The group employed an advanced large-area pixel detector, Pilatus, to obtain the diffraction pattern efficiently. These diffraction data were then treated with an image reconstruction algorithm developed by the team. Several tens of thousands of diffraction images were processed to obtain one super-resolution X-ray image. The algorithm not only reconstructs the sample but also the exact shape of the light probe resulting from the X-ray beam. The 6.8 keV X-ray beam was focused using a zone plate, and the beam size was 300 nm. The spatial resolution achieved was about five times higher. For more information, see the paper, "High-Resolution Scanning X-ray Diffraction Microscopy", P. Thibault et al., Science, 321, 379 - 382 (2008).

Zeolites are microporous crystalline materials, and in the unit cell, the tetrahedrally coordinated Si and Al atoms occupy the so-called crystallographic T-sites. In addition to their pore size, Al's occupancy in the specific T-sites is extremely important in catalytic activity. So far, however, the distribution of Al has remained an unresolved problem. Recently, Professor J. A. van Bokhoven (ETH Zurich, Switzerland) and his colleagues employed the X-ray standing wave technique to study Al distribution in scolecite (CaAl₂Si₃O₁₀-3H₂O, hydrated calcium aluminum silicate). They measured the intensity of X-ray fluorescence, Al K, Si K and Ca Kα near the Bragg conditions of (040), (002) and (-402) reflections. The experiments were done at beamline ID32, ESRF. For more information, see the paper, "Determining the aluminium occupancy on the active T-sites in zeolites using X-ray standing waves", J. A. van Bokhoven et al., Nature Materials, 7, 551-555 (2008).

Progress in nano sciences requires further development of local structural probes, particularly for the study of non-uniform materials. As material functions are often concerned with heterogeneity and some hierarchical orders of the structures, some kind of zooming from low to high resolution will become crucial in the future. Furthermore, in addition to two-dimensional (2D) imaging of an object with a lateral resolution determined by the beam size, some depth resolution is important for a better understanding of materials. So far, X-ray techniques have had several limitations with respect to such points. Recently, French scientists led by Professor J-L. Hodeau (CNRS, Grenoble, France) have reported an interesting development. They are trying to combine pencil-beam tomography with X-ray diffraction to examine unidentified phases in nanomaterials and polycrystalline materials. The experiments were for a high-pressure pellet containing several carbon phases and a heterogeneous powder containing chalcedony and iron pigments. For more information, see the paper, "Probing the structure of heterogeneous diluted materials by diffraction tomography", P. Bleuet et al., Nature Materials, 7, 468 (2008).

X-ray Bragg diffraction can determine crystal structures. So far, however, distinguishing between right- and left-handed crystals has not been done by ordinary X-ray diffraction. Japanese scientists led by Professor S. Shin (RIKEN & The University of Tokyo) recently succeeded in revealing the chirality of crystals by measuring Bragg diffraction near the absorption edge, using circular polarization of synchrotron X-rays at the SPring-8. Reflections only allowed at resonant conditions have been well interpreted for the α-quartz case. For more information, see the paper, "Right Handed or Left Handed? Forbidden X-Ray Diffraction Reveals Chirality", Y. Tanaka et al., Phys. Rev. Lett., 100, 145502 (2008).