Articles

February 1, 2016 02:58 PM

Seifertite SiO2likely exists as a minor phase near the core–mantle boundary. By simulating the pres­sure and temperature conditions near the core–mantle boundary, seifertite was synthesized as a minor phase in a coarse-grained, polycrystalline sample coexisting with the (Mg,Fe)SiO3post-perovskite (pPv) phase at 129 GPa and 2500 K. To date, an unambiguous structure determination of seifertite within its stability field has not yet been achieved due to the challenges in structure studies at megabar pressures.  With recent HPCAT experiments, utilizing a multigrain crystallography approach, the crystal structure of seifertite has been solved and refined based on intensity data collected in-situ at 129GPa. Data sets from six grains having arbitrary orientation have been combined and merged with a reasonable Rfree=0.064.

January 19, 2016 03:05 PM

A novel sample geometry for the laser-heated diamond-anvil cell has been successfully established using nanofabrication techniques, in which a transparent sample layer (SiO2) is sandwiched between two laser-absorbing layers (Ni). The double hot-plate arrangement of the samples, coupled with the chemical and spatial homogeneity of the laser-absorbing layers, addresses problems of spatial temperature heterogeneities encountered in studies where simple mechanical mixtures of transparent and opaque materials were used. With the advanced double hot plate geometry, thermal equations of state for SiO2-stishovite to 50 GPa and 2400 K have been obtained using the on-line laser heating system coupled with in situ synchrotron X-ray microdiffraction at HPCAT. (J. S. Pigott, et al., Geophys. Res. Lett., 42, 10,239-247, doi:10.1002/2015GL066577, 2015)

January 13, 2016 11:48 PM

Coupled degrees of freedom are at the root of the emergent behaviors of functional materials. Through their interconnected responses, electronic, magnetic, and lattice degrees of freedom can manifest physical effects ranging from superconductivity to ferroelectricity to magnetocaloric properties. Recent HPCAT experiments on ferromagnetic LaCo5 show an anisotropic lattice collapse of the c axis near 10 GPa that is also commensurate with a change in the majority charge carriers evident from high-pressure Hall effect measurements. The theoretical calculations predicted that there would be magnetic and electronic transitions accompanying the observed anisotropic lattice collapse. Indeed, associated changes in the electronic structure and magnetic response were measured via high-pressure magnetotransport, which revealed a change in carrier type and a sharp increase in the amplitude of the anomalous Hall effect at 9.3 GPa.

January 8, 2016 01:00 AM

Icosahedrite is the first natural quasicrystal with composition Al63Cu24Fe13, discovered in the Khatyrka meteorite. The quasi-crystalline mineral is found to coexist with high-pressure minerals such as ahrensite and stishovite, indicating its high-pressure formation conditions possibly due to an impact-induced shock. Previous experimental studies on the stability of synthetic icosahedral AlCuFe have either been limited to ambient pressure, for which they indicate incongruent melting at ~1123 K, or limited to room-temperature, for which they indicate structural stability up to about 35 GPa. Recent experiments, using both laser-heated diamond-anvil cells combined with in situ synchrotron X-ray diffraction (at ~42 GPa) and multi-anvil apparatus (at 21 GPa), show the structural evolution and crystallization of possible coexisting phases.

December 4, 2015 05:15 PM

The original concept of “Mott transition” was proposed as the screening of Coulomb potential at high pressures, which has been experimentally validated recently. New HPCAT experiments provide direct experimental evidence of a pressure-induced isostructural Mott transition in cubic perovskite PbCrO3. At the transition pressure of ∼3 GPa, PbCrO3 exhibits significant collapse in both lattice volume and Coulomb potential. Concurrent with the collapse, it transforms from a hybrid multiferroic insulator to a metal. Close to the Mott criticality at ∼300 K, fluctuations of the lattice and charge give rise to elastic anomalies and Laudau critical behaviors resembling the classic liquid–gas transition. Both the large lattice volume and the large Coulomb potential in the low-pressure insulating phase are associated with the ferroelectric distortion, which is substantially suppressed at high pressures.

October 20, 2015 02:14 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 02:08 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 07:03 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 12:20 PM

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