Single-crystal diffraction of (Mg,Fe)SiO3 postperovskite at Mbar pressures
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.  More...

First-Order Liquid-Liquid Phase Transition in Cerium
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.  More...

Anomaly in viscosity correlates with structural changes
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)

X-ray Emission Spectroscopy of Cerium Across the γ-α Volume Collapse Transition
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.

A Pressure-Induced Precursor Lattice Observed in Silicon
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. more....

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