Articles

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.

January 23, 2017 06:56 PM

A metastable liquid may exist under supercooling, sustaining the liquid below the melting point such as supercooled water and silicon. One important question is whether a crystalline solid may directly melt into a sustainable metastable liquid. A research group utilizing HPCAT's facility observed first experimental evidence of creating a metastable liquid directly by a different approach: melting a high-pressure solid crystal of the metal bismuth via a decompression process below its melting point. When a crystal structure of bismuth is decompressed from 3.2 GPa to 1.2 GPa at 489 K, it melts into a liquid at about 2.3 GPa (middle), and then recrystallizes at 1.2 GPa. The decompression-induced metastable liquid can be maintained for hours in static conditions, and transform to crystalline phases when external perturbations, such as heating and cooling, are applied. It occurs in the pressure–temperature region similar to where the supercooled liquid Bi is observed.

January 18, 2017 04:37 PM

Extreme magnetoresistance (XMR) has been recently observed in a growing number of topological semimetals. Recent studies on (W/Mo)Te2 and (Zr/Hf)Te5 suggest that pressure suppresses the XMR and gives rise to superconductivity. To search for a relation between XMR and superconductivity, a research team used HPCAT facility to study the effect of pressure on LaBi. A disappearance of XMR, together with an appearance of superconductivity, is observed at a pressure of ~3.5 GPa, showing the pressure effect on XMR and superconductivity in LaBi. Different from other superconducting XMR materials, a pressure region of coexistence between superconductivity and XMR is observed in LaBi. The suppression of XMR in LaBi is accompanied by a sign change in the Hall coefficient RH from negative to positive. The change in the crystal structure is found to be responsible for the change in band structure and creates a region of band inversion in LaBi.

January 4, 2017 05:43 PM

Spin cross over may be induced by external stimuli such as light, heat, pressure, and magnetic fields, and provides an electronic origin responsible for corporative structural, electrical, magnetic, and/or optical alterations, with wide applications such as in memory, display, and sensor technologies. Based on recent experiments at HPCAT, together with electronic transport data, a research group observed a pressure-driven spin-cross-overs in the 2D honeycomb antiferromagnetic materials MnPS3 and MnPSe3 at room temperature. Applying pressure to the confined 2D systems leads to a dramatic magnetic moment collapse of Mn2+ (d5) from S = 5/2 to S = 1/2. Significantly, a number of collective phenomena were observed along with the spin change, including a large lattice collapse (∼20% in volume), and the formation of metallic bonding.

December 5, 2016 05:05 PM

Strong parallels exist between the packing of tetrahedra and network topologies found in H2O and SiO2 phases. For examples, tridymite and cristobalite are analogues of ice Ih and Ic, respectively; low-energy hypothetical ice analogues of quartz have been predicted theoretically. By loading binary H2 + H2O samples in diamond anvil cells, a team used HPCAT facility and discovered a unique hydrogen-filled, ice-based chiral structure with oxygen topology very similar to the mineral quartz and other, as-of-yet-hypothetical, three-dimensional nets. Structural examination by single crystal x-ray diffraction shows that the H-bonded network forms large channels along the crystallographic c-axis, leaving the 6a Wyckoff position available for rotationally disordered H2 molecules, which is fully consistent with the single-site, filled-ice-like structure inferred from Raman measurements.