Articles

October 20, 2015 01:14 PM

Metallic glasses earn their name from a lack of long-range atomic order and the absence of typical defects, such as dislocations, and exhibit exotic material properties. The incomplete understanding of atomic-level structure in glassy materials has made it challenging to capture the physics of their response to mechanical deformation. Recent HPCAT x-ray diffraction experiments, together with x-ray tomography and molecular dynamics simulations, revealed a crossover between fractal short-range (<2 atomic diameters) and homogeneous long-range structures in metallic glasses. A specific class of fractal, the percolation cluster, is proposed to explain the structural details for several metallic-glass compositions. The results is recently published in Science. (Chen et al, Science, 349, 1306-1310, 2015)

October 20, 2015 01:08 PM

Dye-sensitized solar cells adopting organometal halide perovskites as light absorbers have recently emerged as a promising photovoltaic technology owing to their low material cost and excellent power conversion efficiency. Recent HPCAT experiments have shown an interesting photovoltaic behavior of organolead halides materials under high pressure.

Two phase transformations below 2 GPa (from Pm-3m to Im-3, then to Pnma) and a reversible amorphization starting from about 2 GPa were observed, which could be attributed to the tilting of PbBr6 octahedra and destroying of long-range ordering, respectively. Along with the amorphization process accomplished around 25 GPa, the resistance increased by 5 orders of magnitude while the system still maintains its semiconductor characteristics and considerable response to the visible light irradiation. (Wang et al, JACS, 137, 11144-11149, 2015)

July 7, 2015 06:03 PM

The iron-oxygen system is the most important reference of rocks’ redox state. Even as minor components, iron oxides can play a critical role in redox equilibria, which affect the speciation of the fluid phases chemical differentiation, melting, and physical properties. Until our recent finding of Fe4O5, iron oxides were assumed to comprise only the polymorphs of FeO, Fe3O4, and Fe2O3. Combining synthesis at high pressure and temperature with micro-diffraction mapping at 16-ID-B of HPCAT, we have identified yet another distinct iron oxide, Fe5O6. The new compound, which has an orthorhombic structure, was obtained in the pressure range from 10 to 20 GPa upon laser heating mixtures of iron and hematite at ~2000 K, and is recoverable to ambient conditions.

May 14, 2015 11:20 AM

A recent systematic search for stable calcium carbides was carried out using evolutionary structure prediction calculations.  From ambient pressure up to 100 GPa, six calcium carbide compounds were found to be stable for one or more crystal structures throughout the explored pressure range.  Among the rich and diverse chemistry revealed in these various structures, a particularly remarkable feature was found in the 2D metal phase of Ca2C, specifically, the only known example of a compound in which a metal atom develops a negative Bader charge in the presence of a more electronegative atom.  Researchers led by Tim Strobel from the Carnegie Institution of Washington’s Geophysical Laboratory used HPCAT’s 16-ID-B laser heating end-station to successfully synthesize and subsequently characterize two of these predicted compounds, Ca2C and Ca2C3, for the first time.  A complete account of the research and results, which demon

April 29, 2015 11:56 AM

Certain seismic waves move slower than expected through the Earth’s core, causing researchers to rethink what the innermost region of our planet is made of. One possibility is that the core contains a large amount of carbon. New research on a form of iron-carbide, Fe7C3, shows that it may have the required low seismic wave velocity at high pressure. The experiments, performed at two x-ray beamlines at the U.S. Department of Energy’s Advanced Photon Source (APS), an Office of Science user facility, provide a first-ever estimate of the speed of seismic waves in this iron-carbide at core conditions and suggest that the iron-carbide’s anomalous velocity behavior is due to a change in the electron spin configuration of iron in the material. The results could imply that the Earth’s core is rich in Fe7C3, which might explain where some of Earth’s supposedly “missing carbon” is hiding.

March 20, 2015 05:56 PM

Compressing single-crystal coesite ​SiO2 under hydrostatic pressures of 26–53 GPa at room temperature, HPCAT experiments show a new polymorphic phase transition featured by the formation of multiple previously unknown triclinic phases of ​SiO2 on the transition pathway as structural intermediates. These intermediate phases are accompanied by extensive splitting of the original coesite X-ray diffraction peaks, appearing as dramatic peak broadening and weakening, resembling an amorphous material. This work provides new insights into the densification mechanism of tetrahedrally bonded structures common in nature. (Q.Y. Hu, et al, Nature Communications, 6, doi 10.1038, 2015)

 

March 3, 2015 03:57 PM

As iron is heated, the arrangement of the atoms in the solid changes several times before the iron finally melts.  This unusual behavior is one reason why steel is so strong. The atomic-level details of how and why iron takes on so many different forms during heating remains a mystery, however. Recent work by Caltech CDAC Partner Brent Fultz, current and former Caltech CDAC students Lisa Mauger, Matthew Lucas, and Jorge Muñoz; and HPCAT Beamline Scientists Yuming Xiao and Paul Chow provides evidence for how iron's magnetism plays a role in the melting behavior of iron, and it is this detailed understanding that could help metallurgists develop better and stronger steel.

February 11, 2015 11:16 AM

Energetic radiation can cause dramatic changes in the physical and chemical properties of actinide materials, degrading their performance in fission-based energy systems. The radiation tolerance, referring to the ability to retain atomic structures and properties during irradiation, is of primary concern for the design of nuclear fuels with long operating lifetimes, adequate performance in reactor accident scenarios, and management of wasteforms. While the effect of displacive radiation producing damage through atomic displacement caused by elastic collisions of nuclei has been relatively well studied, the effect of highly ionizing radiation is poorly understood.

February 9, 2015 04:15 PM

Carbon materials are known to possess strong chemical bonds with sp, sp2, and sp3 hybridizations, displaying an impressively rich variety of atomic arrangements, such as graphene, fullerenes, nanotubes, in addition to the well-known crystalline forms of graphite and diamond. These carbon allotropes display distinct mechanical properties, originating from the difference in atomic structures. Searching for new carbon allotropes with unusual physical properties has long been a subject of extensive studies.  Type-II glass-like ​carbon is a widely used material with a unique combination of properties including low density, high strength, extreme impermeability to gas and liquid and resistance to chemical corrosion. It can be considered as a ​carbon-based nanoarchitectured material, consisting of a disordered multilayer graphene matrix encasing numerous randomly distributed nanosized fullerene-like spheroids.