#x ray diffraction

Scientists capture elusive liquid carbon — a diamond in disguise

Research team completes groundbreaking experiment at the European XFEL.

With the declared aim of measuring matter under extreme pressure, an international research collaboration headed by the University of Rostock and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) used the high-performance laser DIPOLE 100-X at the European XFEL for the first time in 2023. With spectacular results: In this initial experiment they managed to study liquid carbon - an unprecedented achievement as the researchers report in the journal Nature. Liquid carbon can be found, for example, in the interior of planets and plays an important role in future technologies like nuclear fusion. To date, however, only very little was known about carbon in its liquid form because in this state it was practically impossible to study in the lab: Under normal pressure carbon does not melt but immediately changes into a gaseous state. Only under extreme pressure and at temperatures of approximately 4,500 degrees Celsius - the highest melting point of any material - does carbon become liquid. No container would withstand that.

Exploring the structures of xenon-containing crystallites

Noble gases have a reputation for being unreactive, inert elements, but more than 60 years ago Neil Bartlett demonstrated the first way to bond xenon. He created XePtF6, an orange-yellow solid. Because it's difficult to grow sufficiently large crystals that contain noble gases, some of their structures—and therefore functions—remain elusive. Now, researchers have successfully examined tiny crystallites of noble gas compounds. They report structures of multiple xenon compounds in ACS Central Science. Since Bartlett's discovery, which is commemorated with an International Historic Chemical Landmark, hundreds of noble gas compounds have been synthesized, and some crystal structures have been characterized by single-crystal X-ray diffraction. However, noble gas-containing crystals are typically sensitive to moisture in air. This chemical property makes them highly reactive and challenging to handle, requiring special techniques and equipment to grow crystals large enough for X-ray diffraction analysis. Therefore, detailed structures of that first xenon compound and several other noble gas-containing compounds have eluded researchers.

Materials Characterization: X-ray Diffraction

X-ray diffraction (XRD) is a technique used for characterizing solid materials, either in powder form or bulk materials. The wavelengths of X-rays (on the order of 10^−8 m to 10^−12 m) are able to diffract directly from lattice planes in crystalline materials (images 2 and 3 above), and the resulting information provided by the scan can be used to collect crystal structure information. The technique can also be used on amorphous materials, but without any order to the atoms or molecules, the result will only be able to confirm the amorphous nature of the material.

However, XRD scans don't simply output a crystal structure. The most basic results are similar to what's shown in image 1 above, with the spots corresponding to lattice planes (an amorphous material would an indistinct ring; see Figure 4 of this link). Another form of the data output is the plot in image 5 above, wherein each peak corresponds to a lattice plane. In order to determine crystal structure from there, one must know the allowed and forbidden reflections for each crystal structure (too detailed to go into here, an example is that the 111 plane is a forbidden reflection in bcc, but allowed for fcc, so if that plane is present it helps narrow down your material). In addition, the height or intensity of the peaks reflects, qualitatively, the frequency of different orientations, thereby providing information about texture.

Though it can be quick and relatively easy to use, with minimal sample preparation, there are limitations of XRD. It works best on single phase materials; XRD can provide information on multiphase materials, and, indeed, provide an idea as to which phases are present if it is unknown, but it is not always reliable. Peak locations must typically be compared to known spectrum and different materials or crystal structures will have overlaps or peaks in the same location.

Sources/Further reading: ( 1 ) ( 2 - images 2 and 3 ) ( 3 ) ( 4 - image 4 ) ( 5 - image 5 ) ( 6 ) ( 7 )

Resolving hydrogen atoms in small organic molecules depending on chemical bond type

Extremely intense X-ray pulses can determine the positions of some hydrogen atoms in organic molecules that form small crystals, an all-RIKEN team has shown. Many areas, including drug discovery and materials research, stand to benefit from this demonstration. Ever since William Lawrence Bragg and his father William Henry Bragg demonstrated that X-rays scattered from crystals produce distinctive patterns 110 years ago, X-ray diffraction has been the technique of choice for determining the structure of crystalline materials. However, many materials form crystals that are too small to be analyzed by X-ray diffraction. "Because many compounds cannot be obtained in large crystals, the ability to analyze the structures of small crystals is important in fields such as synthetic organic chemistry, pharmaceutical science and materials science," says Koji Yonekura of the RIKEN SPring-8 Center. Switching from X-rays to electrons can allow the structures of smaller crystals to be determined, but it has the downside that it requires very thin samples.

Finding hidden regularities in nature: Researchers apply deep learning to X-ray diffraction

X-ray diffraction (XRD) is an experimental technique to discern the atomic structure of a material by irradiating it with X-rays at different angles. Essentially, the intensity of the reflected X-rays becomes high at specific irradiation angles, producing a pattern of diffraction peaks. An XRD serves as a fingerprint for a material since each substance produces a unique pattern.

In research and development, changes in XRDs are used to identify the positions and amounts of additional elements that need to be added to fine-tune a material to help enhance a desired functional property, say, energy storage efficiency in batteries.

However, the peak changes in XRDs are barely discernible to humans. This makes ascertaining the features and relevance of different peaks for material characterization difficult. To this end, a group of Japanese researchers, led by Professor Ryo Maezono from the Japan Advanced Institute of Science and Technology (JAIST), applied a Deep Learning technique called "auto-encoder" to the problem to find hidden regularities in XRDs that could help accelerate the development of new functional materials.

The research team also included Associate Professor Kenta Hongo and Assistant Professor Kousuke Nakano from JAIST. Their work has been published in Advanced Theory and Simulations.

Decoding the structure and properties of near-infrared reflective pigments

Urban areas without sufficient tree cover are significantly warmer than their surroundings. This "urban heat island" effect mainly results from an absorption of near-infrared (NIR) radiation in sunlight. NIR-reflective pigments that can mitigate such heating effects are, therefore, highly desirable.

In particular, functional inorganic pigments are an attractive candidate on this front. In fact, Dr. Ryohei Oka and his colleague from Nagoya Institute of Technology, Japan, have demonstrated that layered perovskite ceramic compounds of the type A2BO4 are ideal for reflecting NIR. In his previous study, it was discovered that novel perovskites such as titanium-added calcium manganese oxide (Ca2(Mn,Ti)O4) ceramics are much better at reflecting NIR radiation than commercially available black pigments. However, the mechanism by which Ca2(Mn,Ti)O4 achieves this remarkable feat remains unknown.

In a recent study published in Inorganic Chemistry, Dr. Oka and his colleague, Dr. Tomokatsu Hayakawa, analyzed the structure and composition of Ca2(Mn,Ti)O4 using a combination of standard theoretical and experimental techniques to investigate the factors contributing to its enhanced NIR reflectivity. This paper was made available online on April 19, 2022, and published in Volume 61 Issue 17 of the journal on May 2, 2022.

How can X-ray diffraction be used for a reliable study of nanostructured materials?

Owing to their unique physical properties, nanostructured materials are now at the forefront of materials science. Several different techniques can be used to characterize their microscopic features, but each of these has its pros and cons. In new research published in The European Physical Journal Special Topics, Jenő Gubicza at ELTE Eötvös Loránd University, Budapest, shows that one indirect method, named X-ray diffraction line profile analysis (XLPA) is suitable for analyzing nanostructured materials, but its application and interpretation require special care for obtaining reliable conclusions.

Nanostructured materials consist of nanoscale grains, each composed of an orderly atomic lattice. Useful properties stem from abrupt changes to the arrangements of atoms in these lattices, named "defects." To fine-tune a nanostructure's material properties, researchers can control the density of these defects by an appropriate selection of the processing conditions of nanomaterials.

To compare the defect densities introduced by both of these approaches, XLPA measures how X-rays are diffracted by the microstructures contained in the materials as they pass through. The concern here is whether the information about defect structure obtained by XLPA is reliable, since this method studies the material indirectly only through the scattering of X-rays. Alternatively, transmission electron microscopy (TEM) can provide extensively detailed images of these microstructures, but can only be used to study tiny volumes.