November 21, 2014 02:32 PM

Extensive experimental studies of nitrogen have revealed a rich and varied phase diagram, with several molecular solids at modest pressures and temperatures, and several molecular, amorphous, and polymeric solids and liquids at ultrahigh pressures and temperatures. Many more phases have been predicted from computational work. Recent attempts to synthesize a novel phase of nitrogen at pressures and temperatures exceeding 125 GPa and 3000 K (well above the synthesis conditions of the polymeric cubic gauche phase) have yielded a single-bonded, layered polymeric nitrogen (LP-N) phase with a remarkable structure and similarly remarkable properties. The new structure is characterized by 3D (cg-N) to 2D transition to the theoretically predicted Pba2 structure, consisting of seven-membered N-N rings.

November 18, 2014 07:35 PM

In order for silicon to be more attractive for use in new technology, its indirect band gap needs to be altered. A new form of silicon has been synthesized with a quasi-direct band gap that falls within the desired range for solar absorption, something that has never before been achieved. The silicon is an open-framework allotrope in the same way that diamonds and graphite are both forms of carbon. Unlike the conventional diamond structure, this new allotrope consists of an interesting zeolite-type structure, which is comprised of channels with five-, six-and eight-membered silicon rings.  The new compound was obtained using a novel high-pressure precursor process. First, a compound of silicon and sodium, Na4Si24, was formed under high-pressure conditions. Next, this compound was recovered to ambient pressure, and the sodium was completely removed by heating under vacuum.

October 20, 2014 10:31 AM

A pressure-volume isotherm in cerium metal at 1100 K was measured in a large volume press of the Paris-Edinburgh type up to 6 GPa. The volume was determined by imaging a rectangular shape of the sample via white X-ray radiography. Energy dispersive x-ray diffraction spectra were recorded to ensure that the highly reactive cerium in the cell assembly remained pure at this temperature. Even at 1100 K the p-V equation of state of liquid cerium shows a pronounced decrease of the bulk modulus above the gamma-phase region similar to the 775 K isotherm in the solid that also shows an inflection point between gamma and alpha-type cerium. The inflection point in the 1100 K isotherm indicating the minimum in the bulk modulus separating the gamma from the alpha-type liquid is located at approximately 3.5 GPa. (M.J. Lipp, et al., J. Phys., Conference Series, 500, 032011, 2014)

October 20, 2014 11:42 AM

The material 2,4,6-trinitrotoluene (TNT) is a molecular explosive that exhibits chemical stability in the molten phase at ambient pressure. The experiments partially performed at HPCAT show that the chemical stability of molten TNT is limited, existing in a small domain of pressure-temperature conditions below 2 GPa. Decomposition dominates the phase diagram at high temperatures beyond 6 GPa. From the calculated bulk temperature rise, it is unlikely that TNT melts on its principal Hugoniot. The work is published in D.M. Dattelbaum, et al, Appl. Phys. Lett., 104, 021911, (2014)

October 17, 2014 05:16 PM

Recently developed rapid compression techniques have been integrated with high temperature furnaces. The entire sample assembly is enclosed in a vacuum chamber (Fig. 1) to avoid oxidation of diamond anvils and maximize the thermal stability. Samples at elevated temperatures can be compressed in a few seconds to megabar pressures. With the fast detector, Pilatus 1M, available at HPCAT, x-ray diffraction from samples and internal standard(s) can be recorded at the millisecond scale, such that thousands of diffraction images can be recorded in one fast loading process. Thus, isothermal compression curves can be obtained in a few seconds.

(Key contributors: Jesse Smith, Stanislav Sinogeikin, Eric Rod, Guoyin Shen)




October 14, 2014 02:15 PM

The occurrence and mobility of carbonate-rich melts in the Earth’s mantle are important for understanding the deep carbon cycle, as well as related geochemical and geophysical processes. Despite extensive studies on the occurrence and stability of carbonate-rich melts, physical properties (such as density, viscosity, and mobility) of carbonate melts are not well understood. Falling sphere viscosity measurement using ultrafast X-ray imaging in Paris-Edinburgh cell at 16-BM-B, HPCAT, revealed that viscosities of carbonate (CaCO3 and dolomite) melts are surprisingly low; in the range of 0.006-0.010 Pa s. These low viscosities are close to that of waterand orders of magnitude lower than those of silicate melts. This implies that the mobility, the ratio of melt-solid density contrast to melt viscosity, of carbonate melts is ~2-3 orders of magnitude higher than that of basalt melts.

July 31, 2014 06:12 PM

It has been known that the Earth’s atmosphere contains 20 times less amount of xenon relative to the lighter noble gases – neon, argon, and kryption, which is referred as ‘missing xenon paradox’. Recent HPCAT experiments shed a new light on this problem.   A zeolite, Ag-natrolite, can absorb and retain 28 wt.% of Xe at 1.7 GPa and 250 ˚C, a condition found in subsurface Earth. Previous studies showed that argon and krypton insertion to Ag-natrolite was reversible via a ‘rotating squares’ mechanism, widening the pores under pressure. However, the behavior of the zeolite is distinct in the presence of Xe – irreversibly inserted and needs heat to desorb even after releasing the pressure. The oxidation state change in Ag, disproportionation of Ag+ into Ag0 and Ag2+, is observed during the insertion and desorption processes, which indicates a possible stabilization of the Xe compound.

May 23, 2014 11:01 AM

The mineralogical constitution of the Earth’s mantle dictates the geophysical and geochemical properties of this region. Previous models of a perovskite-dominant lower mantle have been built on the assumption that the entire lower mantle down to the top of the D″ layer contains ferromagnesian silicate [(Mg,Fe)SiO3] with nominally 10 mole percent Fe. On the basis of experiments conducted at HPCAT using laser-heated diamond anvil cells, at pressures of 95 to

May 13, 2014 12:58 PM

Multilayered transition metal dichalcogenides (TMDs), such as MoS2, fall within a class of 2D materials that may exhibit remarkable optical, electronic, and structural properties depending on the interactions both within and between atomic layers.  A number of “impurity” methods—doping, intercalation, and site defects, for example—have been exploited in an effort to modify the material properties of MoS2.  A group of researchers from the United States, India, and China recently took an alternative approach by applying high pressure to high-purity samples of multilayered MoS2.  Using several complementary experimental techniques, the researchers clearly demonstrate a pressure-induced semiconducting to metallic transition at approximately 19 GPa.  Furthermore, synchrotron powder x-ray diffraction measurements—carried out in part at HPCAT—reveal only a slight lattice distortion across the phase tra