Scientists at Argonne National Laboratory and Ames Laboratory, Iowa State University, have recently performed X-ray magnetic circular dichroism (XMCD) measurements of giant magnetocaloric material, Gd5(Ge1-xSix)4. It was found that germanium becomes magnetic by spin-dependent hybridization between Ge 4p and Gd 5d. This hybridization can change at the germanium-silicon bond-breaking transition, causing the destruction of magnetic ordering and leading to the giant magnetocaloric effect. By combining the experimental results with calculations based on density functional theory, it was concluded that the magnetized germanium orbitals act as "magnetic bridges" in mediating the magnetic interactions across the distant gadolinium ions. For more information, see the paper, "Role of Ge in Bridging Ferromagnetism in the Giant Magnetocaloric Gd5(Ge1-xSix)4 Alloys", D. Haskel et al., Phys. Rev. Lett., 98, 247205 (2007).
Atoms become ions when exposed to extremely intense light. The process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Professor F. Krausz (Max Planck Institute of Quantum Optics in Garching, Germany) and his collaborators recently reported the real-time observation of this most elementary step in strong-field interactions, i.e., light-induced electron tunnelling. The team used 250-attosecond pulses of UV radiation, and confirmed theoretical predictions about the tunneling process. It was also found that the process lasted for several hundred attoseconds, depleting atomic-bound states. This would suggest that the use of tunneling itself is feasible for probing short-lived, transient states of atoms or molecules, e.g., multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes etc. For more information, see the paper, "Attosecond real-time observation of electron tunnelling in atoms", M. Uiberacker et al., Nature, 446, 627 (2007).
It has long been believed that birds can in some way use the natural magnetism of the earth to navigate. Recently, scientists from the University of Frankfurt employed micro X-ray fluorescence as well as micro XAFS spectroscopy to analyze the skin of the upper beak of homing pigeons. Within the skin lining, they established the existence of tiny maghemite (g-Fe2O3) and magnetite (Fe3O4) particles (with a ratio of around 9:1) in the dendritic nerve branches that were arranged in a 3D pattern. According to the research team, this strongly supports the theory that the upper beak of pigeons houses a highly sensitive magneto-receptor that can be used for navigation. The experiments were done with synchrotron X-rays at HASYLAB in Hamburg, Germany. For more information, see the paper, "A novel concept of Fe-mineral-based magnetoreception: histological and physicochemical data from the upper beak of homing pigeons", G. Fleissner et al., Naturwissenschaften, published online in mid-March, 2007.
Platinum is the most efficient electrocatalyst for accelerating chemical reactions in fuel cells for electric vehicles. However, the reactions that take place during the stop-and-go driving of an electric car cause the platinum to dissolve, which reduces its efficiency as a catalyst. Recently, a Brookhaven National Lab group led by Dr. R. Adzic found that adding gold clusters to the platinum electrocatalyst is effective in stabilizing and prolonging the life of the electrocatalyst. The group tested the performance under the oxidizing conditions of the O2 reduction reaction and potential cycling between 0.6 and 1.1 V in over 30,000 cycles, and obtained successful results. X-ray absorption spectra measured at the Pt LIII edge clearly showed that the Au clusters contribute to protecting the platinum from being oxidized. The next step of the research is to duplicate the results in real fuel cells. For more information, see the paper, "Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters", J. Zhang et al., Science, 315, 220 (2007).
In January 2006, the Stardust spacecraft brought back a number of tiny particles from comet Wild 2, which is believed to have originated within a cloud of comets just beyond the orbit of Neptune called the Kuiper Belt. The particles have been analyzed by X-rays at six synchrotron radiation facilities around the world, ESRF (France), APS (Argonne, USA), SSRL(Stanford, USA), ALS (Berkeley, USA), NSLS (Brookhaven, USA) and SPring-8 (Japan). The particles from this comet are important because they are believed to be close to the starting material of the solar system, which is now about 4.5 billion years old. The particles were found to contain a wide variety of minerals and organic materials that look similar to those seen in primitive meteorites found on earth, but the samples also revealed the presence of new materials not previously found in meteorites. It was also discovered that the samples contained minerals similar to Calcium Aluminum-rich inclusions (CAIs), which can be formed at high temperatures, i.e., in the innermost part of the solar nebula, well inside the orbit of Mercury. For more information on the Stardust mission, visit http://stardust.jpl.nasa.gov/home/index.html. Some interesting results have been published as part of a special series of papers in the Dec. 15, 2006, edition of the journal Science.
Professor Weckhuysen (Utrecht University, Netherlands) and his colleagues have recently solved the molecular mechanism for the organic-base-mediated synthesis of zeolites. AlPO4-5 is a typical zeolite, which can be constructed from aluminium-based tetrahedra (AlO4) and phosphorus-based tetrahedra (PO4). The research group compared the formation of the chargeless AlPO4-5 framework with the negatively charged framework (known as ZnAPO-34) that is formed by replacing Al3+ in AlPO4-5 with Zn2+. The former contains one-dimensional channels, but the latter spherical cavities rather than channels. By employing not only small and wide angle X-ray scattering (SAXS and WAXS), but also X-ray absorption spectroscopy, it was possible to observe in real time both the structural changes in the aluminophosphate gel and the conformational features of the organic base (tetraethylammonium hydroxide) used as a template for the crystallization of zeolite. The tetraethylammonium ion was found to form a complex with developing zeolite subunits in the gel, adopting a molecular structure close to that found in the final crystal. This molecular recognition process determines which type of crystal lattice is formed. The principal point here is that molecular organization takes place before crystallization. The experiments were done at BM26A, ESRF (Grenoble, France). For more information, see the paper, "A Combined SAXS/WAXS/XAFS Setup Capable of Observing Concurrent Changes Across the Nano-to-Micrometer Size Range in Inorganic Solid Crystallization Processes", A. M. Beale et al., J. Am. Chem. Soc., 128, 12386 (2006). Another interesting account can also be found in "Physical chemistry: Porous solids get organized", R. A. van Santen1, Nature, 444, 46 (2006).
Artists in ancient Pompeii painted the town red 2,000 years ago with a brilliant crimson pigment made of cinnabar (HgS) that dominated many of the doomed city's wall paintings. The eruption of the volcano Vesuvius showered the neighbouring towns in pumice and ash, and the Villa Sora, in Torre del Greco, remained buried until just 20 years ago, which is when excavation work started. In the remains of the house, the distinctive red colour of the wall frescoes has turned black in many places. The origins of this darkening degradation have not been clearly identified yet and remain a major issue for curators. At ESRF, by aid of micro X-ray fluorescence and absorption spectroscopy, scientists analyzed red cinnabar paintings coated on a sparry calcite (CaCO3) mortar exhibiting different levels of degradation. The results indicate two possible degradation mechanisms; formation of HgCl2 and CaSO4 through reaction with NaCl and SO2 from the environment, respectively. For more information, see the paper, "Blackening of Pompeian Cinnabar Paintings: X-ray Microspectroscopy Analysis", M. Cotte et al., Anal. Chem., 78, 7484-7492, (2006).
Scientists at North Carolina State University, USA recently published an interesting report on the significance of the anomalously high concentration of local Ge-Ge bonds in amorphous Ge2Sb2Te5, which is commonly used in data storage technologies such as DVD, DVD-RAM etc. The ability to change phases from a crystalline to a non-crystalline state is what allows the DVD to take and hold data. While the basic properties of this alloy are well known, there are still a lot of unsolved problems: we do not know how the process works on a microscopic level. The paper indicates that the amorphous phase is an ideal network structure in which the average number of constraints per atom equals the network dimensionality. For more information, see the paper, ""Application of Bond Constrain Theory to the Switchable Optical Memory Material Ge2Sb2Te5", D. A. Baker et al., Phys. Rev. Lett. 96, 255501 (2006).
A series of very interesting experiments has been performed at beamline ID26 at the European Synchrotron Radiation facility (ESRF), Grenoble, to see how gold nanoparticles catalyze carbon monoxide (CO) with oxygen (O2) into carbon dioxide (CO2). High-energy resolution X-ray absorption spectroscopy reveals how the oxygen becomes chemically active when bound to the particles. The reaction took place when the scientists switched from a flow of O2 to one of CO - the O2 bound to the gold reacts with the CO to form CO2. The technique can be applied to a variety of reactions. For more information, see the paper, "Activation of Oxygen on Gold/Alumina Catalysts: In Situ High-Energy-Resolution Fluorescence and Time-Resolved X-ray Spectroscopy", Jeroen A. van Bokhoven et al., Angewandte Chemie, published online June 21, 2006.
Professor E. Ma (Johns Hopkins University, USA) and his colleagues recently succeeded in explaining the atomic packing of metallic glasses, which are of great importance due to their distinctive mechanical and magnetic properties. The structure is known as 'amorphous' (non-crystalline) and shows no sharp Bragg peaks in the X-ray diffraction pattern. The research group adopted quite a unique strategy; first, they aimed at obtaining 3D pictures in the short-to-medium range, unlike conventional atomic-level analysis, which looks only at short-range order, and secondly, they did not resort to a predetermined structural model but used reverse Monte Carlo simulations based on experimental X-ray diffraction and absorption data. One of their key findings was that metallic glass atoms do not arrange themselves in a completely random way. Instead, groups of 7-15 atoms tend to arrange themselves around a central atom, forming 3D shapes called Kasper polyhedra, which join together in unique ways as small nanometer-scale clusters. For more information, see the paper, "Atomic packing and short-to-medium-range order in metallic glasses", H. W. Sheng et al., Nature, 439, 419-425 (2006).
Natural photosynthesis can convert solar energy into chemical energy with almost 100% efficiency. During photosynthesis, O2 is evolved at a tetra manganese-calcium complex bound to the proteins of photosystem II. As the details of the mechanism have not been fully understood, artificial solar systems still capture only a minute amount of energy. Very recently, a German research group has succeeded in providing new insights into the mechanism by means of time-resolved X-ray absorption spectroscopy, which measures Mn K X-ray fluorescence after laser-flash illumination with a time resolution of 10 μs. The model of the so-called S-cycle treats the manganese complex cycles through five oxidation states, but only four intermediates have been identified experimentally (S0 through S3). Dioxygen is formed during the transition from S3 to S0, but the expected S4 intermediate in this transition has been elusive. Real-time X-ray monitoring of photosynthetic O2 production has identified the S4 intermediate and, in contrast to previous proposals, the research group concluded that it is formed by a deprotonation process rather than by electron transfer. The experiments were done at beamline ID26, European Synchrotron Radiation Facility (ESRF), Grenoble, France. For more information, see the paper, "Photosynthetic O2 Formation Tracked by Time-Resolved X-ray Experiments", M. Haumann et al., Science, 310, 1019-1021 (2005).
An international team led by Professor M. Sandström (University of Stockholm) has analyzed the sulfur and iron composition of the wooden timbers of the Mary Rose, a warship of King Henry VIII of England that was wrecked in 1545 and salvaged two decades ago. Synchrotron X-rays from the Stanford Synchrotron Radiation Laboratory (USA) and the European Synchrotron Radiation Facility (France) were employed. The experimental results indicate the surviving wood contains two tons of sulfur in different forms, uniformly distributed within the 280-ton hull. In addition, the Mary Rose contains a great deal of iron from corroded iron bolts, nails and other objects from the ship. Exposed to oxygen in air, the iron catalyzes the oxidation of sulfur into sulfuric acid. For more information, see the paper, "Sulfur accumulation in the timbers of King Henry VIII's warship Mary Rose: a pathway in the sulfur cycle of conservation concern", M. Sandström et al., Proceedings of the National Academy of Sciences of USA, 102, 14165-14170 (2005).
Professor E. Weber's team at Berkeley, California, US has recently succeeded in finding a new technique to handle metal defects in low-grade silicon, which could dramatically reduce the cost of solar cells. At present, around 90 % of solar cells in the world are made from a refined, highly purified form of silicon. This is because solar cells made from cheaper forms of silicon do not perform well and also because removing impurities is expensive. The new idea is to manipulate the impurities in a way that reduces their detrimental impact on the solar cell, instead of purifying the material. The team analyzed how metal contaminants in silicon respond to different types of processing using a synchrotron X-ray microprobe capable of detecting metal clusters as small as 30 nanometers. In addition to micro-XRF and micro-XAFS, they employed a new method based on a spectrally resolved X-ray-beam-induced current, which generates a map of the minority-carrier diffusion length, revealing the precise impacts of metal impurity clusters on local material performance. They found that they were able to manipulate the distribution of the metal impurities by varying the cooling rate of the silicon. When the material is cooled quickly, the metal defects are quickly locked in a scattered distribution. For more information, see the paper, "Engineering metal-impurity nanodefects for low-cost solar cells", T. Buonassisi et al., Nature Materials, 4, 676-679 (2005).
X-ray analysis is a strong tool for speculating on the chemical and physical weathering processes on Mars. The elemental compositions of the bright dust, dark soil and other soil components collected at different sites on Mars, such as the Gusev crater and Meridiani Planum, have been determined by X-ray fluorescence spectrometers fitted to the Mars Explorer Rovers. A comparison between the results obtained at both sites generally shows that the bright dust is global in nature and not dominated by the composition of local rocks, and also that the dark soil has the same origin, while other soil components are fairly different. For more information, see the paper, "An integrated view of the chemistry and mineralogy of martian soils", A. S. Yen et al., Nature, 436, 49-54 (2005).
A joint research group from the Universities of Sheffield and Warwick (both in the United Kingdom) and the European Synchrotron Radiation Facility (ESRF) in Grenoble, France have recently reported an interesting application of a dispersive XAFS (X-ray absorption fine structure) spectrometer, which has no moving parts and is thus inherently more stable than a conventional step-scanning instrument, thereby permitting comparative measurements to be taken rapidly. The measurements were performed on a FeCo alloy thin film located between the poles of a magnet, which induces a saturating field in the sample. The magnets were rotated via a stepping motor such that the induced magnetization, causing the strain, lies either along or perpendicular to the X-ray polarization vector. Transmitted X-ray intensity measurements were made repeatedly at every 90 degree angle between the magnetization vector and the polarization vector. An entire four-quadrant measurement took about 1s, with repeated measurements accumulated over a 2h period. The differential absorption spectra obtained in this way can give atomic displacements due to magnetostriction. The research group demonstrated that it is possible to observe the movement of atoms with a resolution of 0.01 Å i.e., an improvement of 100 times on the previous level. For further details, see the paper, "Measurement of femtometre-scale atomic displacements by X-ray absorption spectroscopy", R. F. Pettifer et al., Nature, 435, 78-81 (2005).
An Austrian research group recently succeeded in obtaining highly collimated, spatially coherent X-rays, at a wavelength of about 1 nm and at photon energies extending to 1.3 keV, from high-order harmonic generation in an atomic gas ionized by a 720-nm, 5-fs, 0.2-TW laser pulse. The beam divergence was evaluated as 0.2 mrad for the spectral range above 200 eV from a knife-edge scan, indicating perfect coherence of the atomic dipoles within a macroscopic volume of diameters of 13μm and 4μm at photon energies of 0.3 keV and 1 keV, respectively. The beam seems to be diffraction-limited to within a factor of five. The spectrum of the generated radiation was observed by an energy-dispersive X-ray spectrometer with some filters. The results are really exciting, because they could detect the copper L-edges (~950 eV)! One would notice that the energy of photons produced by laser technologies has been increasing every year - the main idea behind this progress is the creation of time-gradient in the driving pulse, which allows some 25% of the helium atoms to be ionized within half a cycle before the pulse peak. The electrons detached within this time are pushed in the most intense half-cycle back to the atomic core. For more information, see the paper, "Source of coherent kiloelectronvolt X-rays", J. Seres et al., Nature, 433, 596 (2005). C. Streli and P. Wobrauschek (Atominstitut der Osterreichischen Universtitaten, Technische Universitat Wien) were the co-authors of this paper.
One of the most exciting recent scientific discoveries is that Mars was possibly once wet and salty, suggesting an environment that could serve as a candidate for early life. The two Mars Rovers, Sprit and Opportunity, have been collecting large amounts of data on the soil, rock and atmosphere by utilizing state-of-the-art analytical instruments including an X-ray spectrometer, which recently determined the major and minor elements of soil and rock samples taken from Meridiani Planum. For more information, see the paper, "Chemistry of Rocks and Soils at Meridiani Planum from the Alpha Particle X-ray Spectrometer", R. Rieder et al., Science, 306, 1746-1749 (2004).
A whitish cream in a small canister, which was recently discovered during archaeological surveys of the remains of a Roman temple in London, has been found to contain SnO2. Archaeologists think the SnO2 was added intentionally, presumably for use as cosmetic. They believe the unguent was prepared using sophisticated technology: animal fat was heated, possibly with the aim of bleaching it, and the starch was separated by treatment of roots or grains with boiling water, and then white SnO2 , which is readily produced by heating refined tin metal in air, was added. The non-toxic properties of SnO2 would also have been desirable, because by the second century AD, the dangers of lead were becoming recognized. XRF and XRD analysis played an important role in the identification of the ancient cosmetic cream. For more information, see the paper, "Archaeology: Formulation of a Roman cosmetic", R. P. Evershed et al., Nature, 432, 35-36 (2004).
Argonne research group recently published details of their successful application of high-spatial-resolution XRF and XAFS measurements, which they performed in order to make elemental maps and qualitative chemical analyses of single free-floating, or planktonic, and surface-adhered, or biofilm, cells of Pseudomonas fluorescens. The results revealed differences between the planktonic and biofilm cells in terms of morphology, elemental composition and sensitivity to hexavalent chromium, a heavy-metal contaminant and a known carcinogen. The biofilm cells were more tolerant of the contaminant, which damaged or killed the planktonic cells. The experiments were performed with a 150 nm X-ray beam produced by phase zone plate at the beamline XOR 2-ID, at the Advanced Photon Source (APS), Argonne, USA. For more information, see the paper, "Elemental and Redox Analysis of Single Bacterial Cells by X-ray Microbeam Analysis", K. M. Kemner et al., Science, 306, 686-687 (2004).
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