May 23, 2014 12:01 PM

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 01:58 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

May 9, 2014 03:51 PM

The experimentally observable principal diffraction peak position (q1) is closely associated with the average interatomic distance in the first shell.  The peak position of the principal diffraction is often used to estimate the densities of metallic glasses based on the cube of the one dimensional interatomic distance.  However, by combining high-pressure x-ray diffraction (HPCAT 16-ID-B), ultrasonic sound velocity (HPCAT 16-BM-B) and full field nanoscale x-ray transmission microscopy (APS 32-ID-C) measurements, an unexpected power law is revealed, with the density varying with the 5/2 power of q1, rather than the expected cubic relationship.  Further studies for metallic glasses with different compositions reproduced the same fractional power law.  These observations suggest that the fractional power law of 5/2 may be a universal characteristic of metallic glasses.  [Zeng et al., Phys. Rev. Lett. 112, 185502, (2014)]

March 3, 2014 12:20 PM

In our daily lives we tend to think of electrical conductivity as largely static: Copper is a good choice for conduction; clay is not. But heat up that copper wire, and electron conduction slows. Give a flake of that ceramic a good squeeze, and conduction may perk up. Conductivity is determined by much more than simple chemistry. Metal-to-insulator transitions have excited and perplexed researchers for over a century, and they continue to provide fodder for research today. The key to understanding what causes changes in material conductivity lies in teasing out contributions from structural atomic arrangements and electron interactions. Researchers using high-energy x-rays from the U.S. Department of Energy Office of Science's Advanced Photon Source (APS) have managed to disentangle these components in vanadium sesquioxide (V2O3), an extensively studied model solid.

February 4, 2014 03:04 PM

Understanding the structural response of silicate melts to pressure and composition is crucial in earth and planetary sciences.  The degree of polymerization is a defining characteristic of silicate melt, and it affects the properties of silicate melts (e.g., viscosity, density).  It has been known that viscosity of depolymerized melts increases with pressure consistent with the free-volume theory, while isothermal viscosity of polymerized melts decreases with pressure up to ~3-5 GPa, above which it turns over to positive pressure dependence.  Structural analyses on silicate melts (NaAlSi2O6-CaMgSi2O6 join) at high pressures (performed at HPCAT), in conjunction with molecular dynamics simulations, showed that structures of polymerized and depolymerized melts respond to pressure in distinct manner, resulting in different viscosity and density behavior.  The viscosity turnover in polymerized liquids corresponds to the tet

January 31, 2014 04:20 PM

Magnesite (MgCO3) is an important phase for the carbon cycle in and out of the Earth’s mantle.  HPCAT experiments provide the first experimental evidence for synthesis of magnesite out of its oxide components (MgO and CO2).  Magnesite formation was observed in situ using synchrotron X‑ray diffraction, coupled with laser-heated diamond-anvil cells, at pressures and temperatures of Earth’s mantle.  Despite the existence of multiple high-pressure CO2 polymorphs, the magnesite-forming reaction was observed to proceed at pressures ranging from 5 to 40 GPa and temperatures between 1400 and 1800 K.  This work supports the notion that magnesite is likely the primary host phase for oxidized carbon in the deep Earth.
(H. P. Scott et all, Am. Mineral. 98, 1211-1218, 2013)


January 22, 2014 06:09 PM

Anatase TiO2 is one of the most important energy materials but has poor electrical conductivity. The structural evolution and pressure-induced phase transitions in Nd-doped TiO2 nanomaterials have been studied by in situ synchrotron x-ray diffraction and Raman spectroscopy (both performed at HPCAT).  The TiO2 samples with higher Nb contents are found to have lower bulk modulus. The electrical measurements together with HPCAT experimental results indicate that the enhancement of electron transport under pressure is associated with structural phase transitions. The pressure-induced conductivity evolution provides direct evidence for rationalizing the correlation of packing factors with electron transport in semiconductors.

The work is published in: Xujie Lu et al., J. Am. Chem. Soc. 2014, 136, 419-426

November 26, 2013 07:55 PM

The Earth’spresent layered structure with a metallic core and an overlying silicate mantle would have required a mechanism to separate iron alloy from silicates. Percolation of liquid iron alloy moving through a solid silicate matrix has been proposed as a possible model for core formation. Many previous experimental results have ruled out percolation as a major core formation mechanism for Earth from experimental data obtained from pressure-temperature conditions equivalent to the upper mantle. Until now, experimental results at lower mantle conditions were not possible due to the required ultrahigh pressure-temperaturesand highresolutionimaging technique.

November 13, 2013 05:13 PM

By combining synchrotron Mössbauer spectroscopy and X-ray diffraction, magnetic and structural transitions of the parent compound of iron-based 122 superconductors BaFe2As2 have been studied under high pressure and low temperature conditions. HPCAT experiments showed that the magnetic ordering transition precedes the structural transition in BaFe2Fe2. The pressure-decoupled effect is quite different from the results by chemical-doping where structural transitions always precede or coincide with magnetic transitions. The new finding provides valuable information on the interplay between magnetism and structure in understanding the superconductivity in iron-based compounds.

Wu et al. PNAS., 110, 17263-17266 (2013)