Articles

March 3, 2014 11:20 AM

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 02:04 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 03:20 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 05:09 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 06:55 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 04:13 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)

September 24, 2013 04:41 PM

Pressure-induced amorphization (PIA) in Ta2O5 nanowires is observed at 19 GPa. The resultant amorphous Ta2O5 nanowires show significant improvement in electrical conductivity compared to that in the traditional amorphous compound. The detailed phase transition process is unveiled by in situ synchrotron x-ray diffraction (performed at HPCAT). The pair distribution functions together with Raman spectroscopy, transmission electron microscopy, and first principle calculation, suggest that the amorphization is initiated by disruption of connectivity between polyhedra at the weak-bonding positions along the a-axis. The PIA nanomaterials with improved physical properties hold great promises for numerous future applications.

 

The work is published in: Xujie Lu et al., J. Am. Chem. Soc. 2013, 135, 13947-13953

 

July 18, 2013 10:00 AM

Using synchrotron Mӧssbauer method, recent HPCAT experiments have measured Mӧssbauer spectra of (Mg,Fe)O up to 90GPa. A quantum critical point where the excitation energy becomes zero, causing spin fluctuations, is found at 55GPa. From theoretical calculations, the existence of the quantum critical point at temperatures close to zero affects not only the physical properties of ferropericlase at low temperatures but also its properties at P-T of Earth’s lower mantle.

Figure. Magnetic phase diagram of ferropericlase at high pressures and low temperatures.

 

This work is published in: Lyubutin et al. Proc. Natl. Acad. Sci., 110, 7142-7147(2013)

July 15, 2013 05:07 PM

High pressure plays an increasingly important role in both understanding superconductivity and the development of new superconducting materials. New superconductors were found in metallic and metal oxide systems at high pressure. However, the superconductivity in molecular systems is extremely rare and until now has been limited to charge-transferred salts and metal-doped carbon species with relatively low superconducting transition temperatures.