April 27, 2011 09:56 AM

Pressure dependence of the molar volume of SiO2 glass with helium loading in this work and without helium from literature data. The thick black line represents the equation of states of crystalline polymorphs of SiO2.

April 18, 2011 02:28 PM

To assess the usefulness of high-entropy alloys based on equimolar FeCoCrNi for magnetic application at high temperature, magnetic and vibrational properties of such materials were measured using vibrating sample magnetometry and nuclear resonant inelastic X-ray scattering. The HPCAT experiments show that the Fe partial phonon density of states of FeCoCrNi is similar to that of FeNi which has the same fcc structure. By adding the heavier Pd to the alloy, the mode energy of iron is decreased. The high energy optical modes between 40-50meV in FeCoCrNiAl2 indicated chemical ordering and formation of an Al-rich sublattice, which also confirmed by XRD experiments. High-entropy alloys based on FeCoCrNi were found to be poor candidates for soft magnetic applications, especially at elevated temperatures.
Lucas et al. J. Appl. Phys. 109, 07E307 (2011)

March 16, 2011 12:22 PM

The energy-loss spectrum of 4He at 13.4 GPa.

The electronic structure of crystalline helium has been measured by inelastic X-ray scattering experiments carried out in part at HPCAT, 16-ID-D. The energy-loss spectra reveal a cut-off energy of 23 eV, a sharp exciton, and a continuum of excitations. Spectra were measured over a range of pressures in a diamond anvil cell, and for a range of scattering angles. The experiments identified the energy minimum at the Brillouin zone center and maximum at the zone edge.  Ab-inito calculations of the energy-loss function based on the Bethe-Salpeter and density functional methods enabled direct comparison to measurements and provided a quantitative picture of helium exciton.
* H.K. Mao et al., PRL 105, 186404 (2010).

March 3, 2011 06:29 PM

Recent HPCAT experiments have lead to an observation of an isostructural phase transition with 9.8% volume collapse at ~1.6 GPa between Phases I (ambient pressure) and II of PbCrO3 perovskite.  The small inset plot in the figure refers to the P-V relation for Phase I. The rare isostructural phase transitions (without change of symmetry) induced by high pressures are usually considered to originate from the electronic structural change. In the case of PbCrO3 this seems to be improbable due to specific electronic configurations of ions Cr4+ and Pb2+. The unit cell volume of phase I seems to be abnormally large from the point of view of crystal chemistry and its bulk modulus estimated from P-V data is much smaller than typical value for oxide perovskites.
 Proc. Natl. Acad. Sci., 107, 14026

February 23, 2011 02:00 PM

Ca-III, the first superconducting calcium phase under pressure, wasidentified as simple-cubic (sc) by previous X-ray diffraction (XRD) experiments. In contrast, all previous theoretical calculations showed that sc had a higher enthalpy than many proposed structures and had an imaginary (unstable) phonon branch. By using high-pressure single-crystal XRD and cryogenic high-pressure XRD, the HPCAT experiments demonstrate that Ca-III is neither exactly sc nor any of the lower-enthalpy phases, but sustains the sc-like, primitive unit by a rhombohedral distortion at 300 K and a monoclinic distortion below 30 K. This surprising discovery reveals a scenario that the high pressure structure of calcium does not go to the zero-temperature global enthalpy minimum but is dictated by high-temperature anharmonicity and low-temperature metastability fine-tuned with phonon stability at the local minimum. Published: PNAS vol. 107, No.

November 19, 2010 11:59 AM

Yttrium Oxide (Y2O3) with particle sizes ranging from 5 nm to 1 µm were studied at high pressure using x-ray diffraction and Raman spectroscopy techniques at HPCAT Sectors 16-IDB and 16BMD beamlines at the Advance Photon Source.  Nanometer-sized Y2O3 particles were shown to be more stable than their bulk conterparts, and a grain size-dependent crystalline-amorphous transition was discovered in these materials.   Click here for full article

November 3, 2010 01:04 PM

Closed electron shell systems, such as hydrogen, nitrogen or group 18 elements, can form weakly bound stoichiometric compounds at high pressures. An understanding of the stability of these van der Waals compounds is lacking, as is information on the nature of their interatomic interactions.  Click here for full article