Articles

February 24, 2016 04:04 PM

Under various compression rates (0.03–13.5 GPa/s), kinetics of the B1-B2 phase transition in KCl has been studied in a dynamic diamond anvil cell using time-resolved x-ray diffraction and fast imaging. The recent HPCAT experiments show that the volume fraction across the transition generally gives sigmoidal curves as a function of pressure during rapid compression. Based upon classical nucleation and growth theories (Johnson-Mehl-Avrami-Kolmogorov theories), the fit of the experimental volume fraction as a function of pressure provides information on effective activation energy and average activation volume at a given compression rate. The resulting parameters are successfully used for interpreting several experimental observables that are compression-rate dependent, such as the transition time, grain size, and over-pressurization. [Lin et al., J. Appl. Phys. 119, 045902, 2016]

February 24, 2016 04:00 PM

A novel phase of nitrogen, λ-N2, has been synthesized through low temperature compression. The new phase exhibits an exceptionally wide range of pressure stability from below 1 to 140 GPa, overlapping nine other known phases. On heating, its transformations are different to those observed in other phases, implying that the phase nitrogen adopts depends not only on P-T path, but also on the initial structural configuration, which greatly complicates its phase diagram. Although the discovery of this new phase by performing compression at low temperature reconciles differences in experiment and theory, the P-T conditions under which λ-N2 is the thermodynamic equilibrium are unknown and pose an interesting challenge for future theoretical studies.

February 12, 2016 03:16 PM

Some metallic glasses follow a non-cubic power law in which its volume contracts as V ~ (1/q1)2.5 with increasing pressure, where q1 represents the principal diffraction peak position. A recent work performed partly at HPCAT, reveals that the 2.5 power law holds even across the first-order polyamorphic transition of a Ce68Al10Cu20Co2 metallic glass under high-pressure. As this transition is equivalent to a compositional change from 4f-localized Ce (big) to 4f-itinerant Ce (small), the result signifies the commonality of the fractal dimension over wide variety of composition and density of metallic glasses. This general 2.5 power law brings insight into the structure of metallic glasses, which is inevitably modified during the pressure and/or composition tuning.

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)