April 4, 2013 09:30 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 06:12 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 01:30 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 05:45 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.

November 5, 2012 05:14 PM

At high pressures, calcium (Ca) not only displays very rich structural changes but also holds the record for the highest superconducting critical temperature (Tc = 26 K) among elemental materials; furthermore Ca is a very active test case of modern theories and experiments, where a number of paradoxes arise based on density functional theory calculations related to the stability of its simple cubic phase. Using synchrotron high-pressure x-ray diffraction at cryogenic temperatures, we have established a new phase diagram for calcium up to 110 GPa and 5-300 K. We discovered the long-sought for theoretically predicted β-tin structured calcium with I41/amd symmetry at 35 GPa in a small low temperature range below 10K, thus resolving the enigma of absence of this lowest enthalpy phase.

Fig. 1 Proposed phase diagram of Ca at high pressure and low temperature.

August 3, 2012 10:37 AM

Research at the U.S. Department of Energy Office of Science’s Advanced Photon Source (APS) by researchers from Argonne National Laboratory, the Brazilian Synchrotron Light Laboratory, the Carnegie Institution of Washington, and Commissariat à l'énergie atomique-CEA (France) has shown that gaining valence information requires a lot more work when it comes to mixed-valence systems. Their results have been published in the journal Physical Review Letters. (more...)

July 23, 2012 12:15 PM

A Carnegie scientist's observations have led the way to stabilizing tungsten hydrides under high pressure. (more...)

July 2, 2012 03:33 PM

A collaborative experimental effort with Carnegie researchers has discovered unknown properties of a computer memory material that will allow for faster data transfer with a higher capacity of data stored. (more…)(