Articles

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)

September 24, 2013 05:41 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 11:00 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 06:07 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.

April 8, 2013 01:37 PM

The (Mg,Fe)SiO3 crystallizes into a postperovskite (pPv) phase in polycrystalline form at high pressure-temperature conditions and becomes amorphous upon release of pressure.  Structural refinement of the pPv phase at extreme conditions is challenging because of uncertainties in diffraction intensity caused by texturing of the sample and spotty diffraction patterns due to crystal growth under high temperature.  Using a newly developed multigrain single-crystal X-ray diffraction analysis technique in a diamond anvil cell, recent HPCAT experiments have obtained crystallographic orientations of over 100 crystallites at high pressure in a coarse-grained polycrystalline pPv sample. A few selected pPv crystallites have been selected and tracked using conventional single-crystal structural analysis and refinement methods.

April 4, 2013 10:30 AM

The liquid polymorphism is of great importance to the understanding of the liquid.   Experimental observation is exceedingly challenging, which involves simultaneous high pressure-temperature conditions, instantaneous capture of the diffuse scattering from liquid phases, and the demarcation of different structural forms of liquid states lacking long-range periodicity.  Recent HPCAT experiments successfully identified a liquid-liquid phase transition in the monatomic liquid metal cerium, by in situmeasuring high-pressure high-temperature x-ray diffraction. At 13 GPa, upon increasing temperatures from 1550 to 1900 K, a high-density liquid transforms to a low-density liquid, with a density change of 14%. Theoretical results suggest that the transition primarily originates from the delocalization of felectrons and is deemed to be of the first order that terminates at a critical point.

March 4, 2013 07:12 PM

Viscosity is one of the most fundamental transport properties in liquid.  Recent HPCAT development of high-speed x-ray radiography combined with a Paris-Edinburgh cell enabled viscosity measurements of low viscos (<1 mPa s) liquids and fluids.  A falling sphere technique revealed an anomaly in the viscosity of liquid KCl at around 2 GPa.  Structural data of liquid KCl showed a pronounced change signified by the ratio r2/r1, where r1 and r2 are the nearest- and the second-neighbor distances, respectively. The results suggest that the viscosity anomaly in liquid KCl strongly correlates with the structural changes. The integration of the viscosity and liquid structure measurements opens a new way for further understanding the dynamics of liquids at high pressures.
(See Kono et al., Phys. Rev. B 87, 024302, 2013)

January 28, 2013 02:30 PM

A team of researchers from Lawrence Livermore National Laboratory, Stanford University and HPCAT have used the X-ray emission spectrometer at 16 ID-D of HPCAT to study Lγ emission of Ce metal across γ-α volume collapse transition which often serves as a testing ground for theoretical models treating f-electron correlations. The satellite peak of Lγ decreases 30% across the volume collapse. The HPCAT experimental results and new dynamical mean field theory (DMFT) calculations provided not only solid evidence to support the Knodo model in conjunction with previous measurements, but also a general experimental methodology to study relevant strongly correlated f-electron systems. Lipp et al. Phys. Rev. Lett., 109, 195705 (2012)

 

 

November 29, 2012 06:45 PM

Silicon is abundant in nature and arguably the most widely used material nowadays. Recent HPCAT experiments show that silicon displays an intriguing precursor lattice at high pressure, which provides a clue for understanding the process and mechanism of phase transitions in solids.  The results from high-pressure single crystal diffraction show that an embryonic phase can dynamically co-exist with the host lattice through collective motions. This collective mechanism for the phase transition goes beyond previously considered reconstructive or displacive processes and provides a novel picture of the underlying dynamics. This work opens a new avenue for exploring precursor phenomena in phase transitions which may be more common than previously thought.