Researchers from the group of CDAC Academic Partner Rod Ewing at the University of Michigan and collaborators from the Geophysical Laboratory have demonstrated a new method to detect and quantitatively measure pressure-induced atomic disordering in pyrochlore oxides. This discovery provides new insight on the formation of of defects at extreme conditions and reveals a possible mechanism for photochemical reactions in these oxides. Pressure-induced structural disordering, such as the substitution of elements or the formation of vacancies and other defects, is very common and is important to earth and material sciences. However, quantitative analysis of disordering at high pressure at the atomic scale is a very challenging problem from technical standpoint. In the particular case of pyrochlore oxides, cations and anions will form antisite defects and Frenkel pairs simultaneously at extreme conditions.
With x-ray diffraction data collected on the pyrochlore oxide La2Zr2O7, at Beamline X17C at the NSLS, the group has shown that defects originating from the cation and anion sites in the pyrochlore structure have different influences on the individual diffraction peaks and their contribution can be quantitatively distinguished in the x-ray diffraction pattern. Using the Rietveld refinement method, it was concluded that anion defects formed below 5 GPa and cation antisite defects dominated above 10 GPa (Fig. a). An anomalous lattice expansion was confirmed in the oxide at 10 GPa by x-ray diffraction, Raman scattering and synchrotron infrared absorption conducted at the NSLS-U2A, if a water-containing pressure medium is used (Fig. b). Water intercalation in pyrochlore oxides may be common during the process of either pressurization or ion irradiation, and it is mainly due to the formation of cationic defects. Some pyrochlore oxides, such as La2Zr2O7, have been used previously as catalysts for the photochemical splitting of water, a process that may be closely related to the formation of defects in the pyrochlore structue [Zhang et al., Phys. Rev. Lett., 105, 015503 (2010)]. |
CDAC student Walter Uhoya (University of Alabama – Birmingham) and colleagues from ORNL, LLNL, and the University of Alabama, have shown that the structure of iron-based superconductor EuFe2As2 expands under pressure. In a paper published in the Journal of Physics: Condensed Matter, Uhoya, et al. report structural studies at HPCAT using designer diamond anvil technology for electrical resistance measurements under pressure. EuFe2As2 crystallizes on a tetragonal lattice at ambient conditions and contains FeAs4 tetrahedral layers in the structure. X-ray studies reveal a surprising increase in the d-spacings of some of the Bragg peaks with pressure, as compared to the decrease in d-spacings that would normally be expected under compression.
 EuFe2As2 forms a collapsed tetragonal structure above 8.5 GPa, and shows normal compression behavior above this pressure. The group also investigated the onset of superconductivity under pressure using designer diamond anvils (see figure) and showed that the superconducting transition temperature (Tc) increases rapidly from 22 K to 41 K within the region of negative compressibility. With an increase in pressure beyond 10 GPa in the collapsed tetragonal phase, the transition temperature is lowered. The group has further shown that this negative compressibility phenomenon is common to iron-based superconductors of type MTM2As2 (M = Ba, Ca, Sr, Eu and TM= Fe, Co) results provide important input to the theory of superconductivity in iron-based superconducting materials.
[Uhoya et al., J. Phys.: Condens. Matt., 22, 292202 (2010)].
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***Registration Deadline August 18, 2010***
For more information, follow this link. |
The May/June, 2010 edition of Synchrotron Radiation News focused on high pressure research. Both of the technical reports in the edition showcased CDAC supported work and were coauthored by CDAC supported beamline personnel, including Zhenxian Liu, Guoyin Shen, Yang Ding, and Wenge Yang, as well as CDAC Academic Partner Tom Duffy (Princeton). The work of (Brookhaven/NSLS) and colleagues from Brookhaven, SUNY, Princeton, and Carnegie was featured in a report about high-pressure research at NSLS. High-pressure research at the APS was the focus of a report by Shen and colleagues from HPCAT, the University of Chicago, APS, and HPSynC.
Figure Captions
Top: Newly established U2A side station with Bruker Vertex 80v FTIR spectrometer and Hyperion 2000 IR microscope.
Center: The plot indicates pressure dependence of far-infrared absorption spectra of H2O and D2O ice VIII obtained at 85 K.
Bottom: Collapse of magnetism measure by NFS and its impact on sound velocity in Fe3S derived from NRIXS data: (a) partial phonon density of states; (b) collapse of magnetism.
- To view these reports and the rest of the high pressure edition of Synchrotron Radiation News, go here.
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An accurate measurement of the pressure is a fundamental aspect of all high pressure experiments, and there is a need for continuous development of new and better pressure scales. Pressure calibration has generally relied on the experimentally-determined equations of state or spectroscopic properties of internal standards such gold or ruby. In work appearing recently in Physical Review Letters, a research team that includes the Geophysical Laboratory’s Ronald E. Cohen, along with scientists from the University of Illinois at Urbana-Champaign, the University of California – Berkeley,  and the Cavendish Laboratory has established a new pressure scale based on a high-accuracy solution to the underlying equations of quantum mechanics, which governs all material properties. In applying this first-principles approach, the group removed a key approximation employed in previous simulations, which has given their simulations an accuracy that rivals experiment.
The group developed an all-electron quantum Monte Carlo (QMC) method for solids that does not rely on pseudopotentials, and used it to construct a primary ultra-high-pressure calibration based on the equation of state of cubic boron nitride. The static contribution to the free energy was computed with the QMC method, and the phonon contribution was then obtained using density functional theory, which yields a high-accuracy calibration up to 900 GPa, which is directly applicable to experiments. The anharmonic Raman frequency shift was also computed with QMC simulations as a function of pressure and temperature, allowing optical pressure calibration. This all-electron method is applicable to first-row solids, providing a new reference for ab initio calculations of solids and benchmarks for pseudopotential accuracy [K. Esler et al., Phys. Rev. Lett., 104, 185702 (2010)]. |
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The 2010 Carnegie Summer Scholars presented the results of their summer's work in a symposium held August 14. Topics included mineral physics, seismology, astronomy, organic geochemistry, high pressure chemistry and studies on the origins of life.
Three Carnegie/CDAC supported post-docs have accepted positions in the National Labs (pictured clockwise above). Raja Chellappa joined LANL in May. Luke Shulenburger will start at SNL in July and Amy Lazicki will begin work at LLNL in September.
Carnegie's Yufei Meng won a Best Poster award at the 4th International Conference on New Diamond and Nano Carbons, which was held in Suzhou, China. Of the 169 poster entries, only 5 were given this honor. Meng received her award from Jilin University's Professor Bingbing Liu (above, left). Photographs of CVD diamonds from her poster, "Large high purity single crystal CVD diamond," include this one of a 2.4 carat diamond that Meng recently synthesized.
Dmitry Popov, a researcher at the Paul Scherrer Institute in Switzerland, joined HPCAT as Beamline Scientist on April 15th.
Gordon Leslie Davis, former Carnegie Staff Member, passed away on 15 February. He was 97 years old. Dr. Davis worked on radium in basalt and was involved in the national defense effort in World War II. For more information about Dr. Davis, please go here
CDAC Academic Partner Robert Downs was elected a fellow of the American Association for the Advancement of Science (AAAS).
