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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)]. |
Two new studies point to the important role that pressure plays in understanding and creating new ferroelectric materials. Lead-based relaxor ferroelectrics with complex perovskite structures exhibit a strong frequency-dispersive dielectric permittivity with a broad and smooth temperature dependence. Their high permittivity and high piezoelectric constants make them suitable for applications in devices for sonar or medical imaging. While extensive theoretical and experimental studies have advanced our understanding of relaxors, their properties are still poorly understood. These difficulties stem from the complexity of these materials, which have a high degree of compositional, structural, and polar disorder.
Using a combination of high pressure synchrotron x-ray diffraction, Raman and Brillouin scattering techniques, Muhtar Ahart and colleagues from Carnegie and Sandia National Laboratory report on experiments revealing that the pressure-induced relaxor-to-ferroelectric crossover in disordered Pb(Sc0.5Nb0.5)O3 is a typical example of such phenomena in disordered perovskite systems and that such behavior can be attributed to the relaxation of polar nanoregions in these systems. The pressure-dependent Raman bands are broad, which is a relaxor-specific spectral signature, and indicate a structural phase transition. A significant softening in the longitudinal acoustic mode is also observed. The similarity in relaxation times found in the temperature and pressure dependencies show that the softening of the longitudinal acoustic mode may be attributed to electrostrictive coupling between polar nanoregions and acoustic modes.
X-ray diffraction reveals that the material is characterized by two distinct pressure-volume compression curves below and above the phase transition pressure.This work is a tribute to co-author George Samara from Sandia. He was a leading figure in the field of ferroelectrics and was the first to demonstrate the importance of the pressure variable in understanding these important materials. He helped launch this CDAC collaboration but passed away in 2006. [M. Ahart et al., J. Appl. Phys., 107, 074110 (2010)].
In a related study, P. Ganesh and collaborators from Carnegie, NIST, Argonne, and Simon Fraser University showed that correlations between chemically ordered regions, which act as polar-nano regions, are responsible for the relaxor behavior in complex relaxors and relaxor ferroelectrics such as Pb(Mg1/3Nb2/3)O3 and Pb(Sc1/2Nb1/2)O3, which are technologically important due to their colossal dielectric and/or piezoelectric response in their relaxor phase. The origin of the so-called relaxor phase in these materials, which show a frequency-dependent dielectric constant that deviates from the Curie-Weiss temperature behavior of normal ferroelectrics, has been a topic of debate for nearly half a century. A joint theoretical and experimental study was performed to explain the origin of the characteristic  diffuse scattering observed in these materials in their ferroelectric, relaxor and paraelectric phases and investigate their temperature and pressure dependence. The findings suggest a possible route to engineer superior dielectric/piezoelectric materials by nano-engineering different types of chemically ordered regions that will enhance the correlations between the polar-nano regions [P. Ganesh et al, Phys. Rev. B, 81, 144102 (2010)]. |
The annual March meeting of the American Physical Society was held in Portland, Oregon from March 15-19, with CDAC researchers well represented among a number of sessions. Seven of the CDAC academic partner groups, along with Carnegie personnel, National Laboratory Partners and HPCAT/HPSynC staff presented 46 papers in 20 different sessions throughout the course of the meeting. Of particular interest was a series of four high pressure research focus sessions, in which the work of a wide cross-section of the CDAC community was included. Of the 45 presentations in these special focus sessions, 20 of them contained a CDAC component in the results. The scientific and technique development work of CDAC Academic Partner Yogsh Vohra's group (Alabama-Birmingham) was featured twice in the focus sessions, and the group of new Academic Partner David Cahill (Illinois) also gave several presentations.
CDAC Scientists from Carnegie presented 9 talks. Maddury Somayazulu and Tim Strobel discussed their experimental work on the high-presure behavior of the Xe-H2 and SiH4-H2 systems, respectively. On the theoretical side, J. G. O. Ojwang and Luke Shulenburger presented new results on transition metal oxides and P. Ganesh gave a talk on first-principles studies on ferroelectric materials. Also representing Carnegie was Angela Schad, a 2009 Carnegie Summer Scholar from the University of Notre Dame, who presented her theoretical work on Raman spectra of ferroelectric oxides.
CDAC Laboratory Partners Dana Dattlebaum (LANL), Will Evans (LLNL) and Dan Dolan (Sandia) also also presented work from their respective high pressure groups. Dattlebaum spoke about the
pressure-induced decomposition of hydrogen peroxide, while Dolan presented work on the P-T phase diagram of tin as determined from dynamic isentropic compression experiments. Evans was a co-author on four talks, and also chaired an important session that promoted new research and employment opportunities at the National Labs. |
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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).
Carnegie CDAC-Affiliated Scientist Yingwei Fei was elected a Fellow of the American Geophysical Union on 22 January. The honor is given to those who have attained "acknowledged eminence in the Earth and space sciences." Only one in each thousand members is elected to Fellowship each year.
