Articles

June 26, 2017 04:52 PM

Accurate pressure scales remain a challenging subject in compression science. Intercomparison of the pressure standards provides important information on their accuracies and reconciles discrepancies between experimental results based on different pressure scales. In order to intercalibrate the equations of state (EOSs) of the three widely used pressure standards, gold, platinum, and MgO, a research group has measured their unit cell volumes using the laser-heated diamond anvil cell up to 140 GPa and 2500 K. The simultaneous measurements of Au+MgO and Pt+MgO allow to compare the EOSs of Au, Pt, and MgO directly at high P-T and further refine them for consistency. At 300 K, three standards agree with each other within ±2.5 GPa to 135 GPa in quasi-hydrostatic media. Refined EOSs have been obtained at 300 K, making the measured results consistent with each other within ±1 GPa up to 135 GPa.

June 23, 2017 06:58 PM

For decades it has been believed that lithium, the simplest metallic element, has a complicated ground-state crystal structure. Using synchrotron x-ray diffraction at HPCAT and multiscale simulations with density functional theory and molecular dynamics, a research group has shown that the previously accepted martensitic ground state is metastable. The new ground state is face-centered cubic. The group shows that different isotopes of lithium (6Li and 7Li) display crystal phase transitions at slightly different pressures and temperatures under similar thermal paths, which could be related to large quantum mechanical effects between the isotopes. Lithium is an extremely challenging material for high-pressure studies: It reacts chemically with many materials (e.g., it causes gasket metals and diamonds to become brittle), and diffraction experiments are challenging because of low scattering cross sections.

June 9, 2017 07:28 PM

Carbon gives rise to remarkable classes of materials with combined properties, such as low weight, high strength, hardness, elasticity, and tunable electronic properties, because of the flexibility to form sp-, sp2-, and sp3-hybridized bonds. A research team succeeded to recover an ultrastrong carbon after compressing glassy carbon at various pressure and temperature conditions.  The newly created glassy carbons are composed of mixed sp2-sp3 hybridized carbons with a low sp3 component, which gives rise to the unique combination of properties.  The compressed glassy carbons have extraordinary compressive strengths and simultaneously exhibit robust elastic recovery in response to local deformations. This type of carbon is an optimal ultralight and ultrastrong material for a wide range of multifunctional applications.   More in Hu et al. (2017), Science Advances, 3: e1603213, DOI: 10.1126/sciadv.1603213.

April 17, 2017 05:42 PM

Zirconium has a low neutron cross section and high degree of corrosion resistance. It is widely used in industry and engineering, in particular in the area of refractory applications. Knowledge of elastic and thermal properties of zirconium is important for understanding the behavior and performance of the materials at extreme conditions.

March 14, 2017 02:55 PM

It has been predicted that hydrogen-rich material may promote metallization of hydrogen through chemical pressure imposed by the foreign atoms or molecules. A group utilizing HPCAT’s facility studied a hydrogen rich compound, Ar(H2)2, to ultrahigh pressures over three megabars. Ar(H2)2 is a typical van der Waals compound, in which Ar atoms and H2 molecules are ‘glued’ together by the London dispersion forces, leaving the H2 units preserved.

March 13, 2017 06:45 PM

As an archetypal semimetal with complex and anisotropic Fermi surface and unusual electric properties (e.g., high electrical resistance, large magnetoresistance, and giant Hall effect), bismuth (Bi) has played a critical role in metal physics. For over a century, Bi has been known to be diamagnetic. A research team observed unusual ferromagnetism in bulk Bi samples recovered from a molten state at pressures of 1.4–2.5 GPa and temperatures above 1,250 K. Using HPCAT facility, the ferromagnetism is found to be associated with a surprising structural memory effect in the molten state. The ability for solid Bi to remember liquid structure motifs seems to be related to structural similarities between the solid and liquid around 2 GPa, where melting temperature is pressure-independent (i.e., no discontinuity in specific volume or density across the melting line).

March 9, 2017 05:32 PM

Meteorites preserve evidence of processes ranging from the formation of the solar system to the origin of life on Earth and the potential for extraterrestrial habitability. One mineral of particular interest in meteorites is the phosphate mineral merrillite, an anhydrous end-member of the merrillite–whitlockite solid solution series, with whitlockite being the hydrogenated end member. Recent experimental observations have raised a question as to why merrillite rather than whitlockite forms in a melt with available H2O at the time of phosphate crystallization. One possibility is that shock has devolatilized what was, in part or whole, whitlockite into merrillite. A research group, using synchrotron facilities including HPCAT, inspected the shock-transformation products of Mg-whitlockite. Post-shock samples show that merrillite is produced from whitlockite (Figure) during experimental shock events.

March 2, 2017 12:27 PM

High-pressure melting anchors the phase diagram of a material, revealing the effect of pressure on the breakdown of the ordering of atoms in the solid. An important case is molybdenum (Mo), which has long been speculated to undergo an exceptionally steep increase in melting temperature when compressed. On the other hand, previous experiments lacking reliable melting criteria, showed a nearly constant melting temperature as a function of pressure, a large discrepancy with theoretical expectations. Using microstructure to define a material’s molten history, a group in recent HPCAT experiments found a high-slope melting curve in Mo. Synchrotron X-ray diffraction was used to analyze the crystalline microstructures, generated by heating and subsequently rapidly quenching samples in a laser-heated diamond anvil cell.

February 14, 2017 06:52 PM

At ambient condition, the chemistry of water is so stable that it is considered as the unit of hydrogen carrier in the hydrosphere. For a long time, cycling of hydrogen is equivalent to the cycling of water in the Earth’s solid crust and mantle. Geoscientists rarely treat water in its decomposable forms, like hydrogen plus oxygen. Using HPCAT facility, a team discovered hydrogen was freed from hydrous minerals in its elemental form. Starting from a typical hydrous mineral (goethite) and placing it under lower mantle pressure-temperature conditions, they have observed hydrogen released and the hydrogen loss closely related with the heating temperature and duration. The released hydrogen can directly source the liquid outer core, which is considered to have a substantial amount of hydrogen.