Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F26: Focus Session: Materials in Extremes: High Pressures and Temperatures |
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Sponsoring Units: GSCCM DCOMP DMP Chair: Anatoly Belonoshko, KTH Royal Institute of Technology Room: 502 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F26.00001: A multiphase equation of state for carbon addressing high pressures and temperatures Lorin Benedict, Kevin P. Driver, Sebastien Hamel, Burkhard Militzer, Tingting Qi, Alfredo A. Correa, Eric Schwegler We present a 5-phase equation of state (EOS) for elemental carbon. The phases considered are: diamond, BC8, simple-cubic, simple-hexagonal, and the liquid/plasma state. Free energy models for the various phases are constrained by Density Functional theory (DFT) and path integral quantum Monte Carlo calculations. The precise manner in which the ideal gas limit is reached is greatly constrained by both the highest temperature DFT data and the path integral data, forcing us to discard an ion-thermal model we had used previously in favor of a new one. Predictions are made for the principal Hugoniot and the room-temperature isotherm, and comparisons are made to recent experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F26.00002: Lattice dynamics and thermal equation of state of cubic CaSiO$_{3}$ perovskite Tao Sun, Renata Wentzcovitch CaSiO$_{3}$ perovskite (CaPv) is believed to be the third most abundant mineral in the Earth's lower mantle and is a major component of mid-ocean ridge basalt (MORB). A well constrained thermal equation of state for CaPv is key to several geophysical problems, e.g., lower mantle composition, density contrast between mantle and plates, nature of D'' region, etc. Its experimental and theoretical determination have been very challenging because the cubic structure that CaPv adopts at lower mantle conditions is unstable at low temperatures and some of its harmonic phonons have imaginary frequencies. We have used a recently developed hybrid method combining ab initio molecular dynamics with vibrational normal mode analysis to compute its free energy and thermal equation of state at lower mantle conditions. These results are essential to understand the fate of subducted MORB in the mantle. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F26.00003: QMC Benchmarks of Density Functionals for High-Pressure Hydrogen Applications Raymond Clay, Jeremy McMinis, Jeffrey McMahon, Carlo Pierleoni, David Ceperley, Miguel Morales It has recently been shown in high-pressure hydrogen that the predicted locations of the liquid-liquid phase transition and the solid insulator-to-metal transition are very sensitive to the choice of density functional employed. We use Quantum Monte Carlo to benchmark some of the most commonly used DFT functionals for dense hydrogen in these two regions of the phase diagram. We find that in both of these phases, van der Waals and hybrid functionals noticeably outperform LDA and PBE functionals, and recommend the use of the vdW-DF [M. Dion \textit{et al.}, Phys. Rev. Lett. \textbf{92}, 246401 (2004)] functional for structural relaxation and molecular dynamics. We look at the impact of the functional on enthalpies, bond lengths, and the location of the liquid-liquid phase transition. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F26.00004: Computation of the Principal Deuterium Hugoniot with quantum Monte Carlo Norm Tubman, David Ceperley We have performed extensive calculations of the principal deuterium Hugoniot using the Coupled Electron Ion Monte Carlo method (CEIMC). In this method we use Monte Carlo to simulate both the electronic and ionic degrees of freedom. We will discuss finite size effects, effects of zero point motion of the ions, as well as convergence issues with the simulation. We compare the predicted Hugoniot with previous simulations and experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F26.00005: Intermediate-spin ferrous iron in the Earth's lower mantle? Han Hsu, Renata Wentzcovitch Using density functional theory $+$ self-consistent Hubbard U (DFT$+$Usc) calculations, we investigate intermediate-spin (IS) ferrous iron (Fe$^{\mathrm{2+}})$ in major lower-mantle minerals, ferropericlase (Fp) and magnesium silicate (MgSiO$_{\mathrm{3}})$ perovskite (Pv). In both minerals, two distinct types of IS Fe$^{\mathrm{2+}}$ are found. In Fp, while both types of IS Fe$^{\mathrm{2+}}$ are configured t$_{\mathrm{2g}}^{\mathrm{5}}$ e$_{\mathrm{g}}^{\mathrm{1}}$, one has a d$_{\mathrm{z2}}$ electron$_{\mathrm{\thinspace }}$and the other has a d$_{\mathrm{x2-y2}}$ electron, referred to as the IS(z$^{\mathrm{2}})$ and IS(x$^{\mathrm{2}}-$y$^{\mathrm{2}})$ state, respectively.$_{\mathrm{\thinspace }}$The IS(z$^{\mathrm{2}})$ state has an exceptionally high QS ($\ge $ 5.5 mm/s); the IS(x$^{\mathrm{2}}-$y$^{\mathrm{2}})$ state has a quite low QS (\textless 0.5 mm/s). Also, the IS(z$^{\mathrm{2}})$ state has a stronger on-site Coulomb interaction and much higher energy. In Pv, while Fe$^{\mathrm{2+}}$ substitutes Mg in the dodecahedral site, it is effectively under a distorted octahedral crystal field, and the two IS states can be characterized by their filled e$_{\mathrm{g}}$-like orbitals as well. These two IS Fe$^{\mathrm{2+}}$, in contrast to those in Fp, are energetically competitive, and they both have a small QS (\textless 1.6 mm/s). Our calculations show that all IS Fe$^{\mathrm{2+}}$ in lower-mantle minerals are unfavorable, and their QSs are all inconsistent with experiments. Therefore, IS Fe$^{\mathrm{2+}}$ is highly unlikely in the Earth's lower mantle. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F26.00006: Predicted novel hydrogen hydrate structures under pressure from first principles Guangrui Qian, Andriy Lyakhov, Qiang Zhu, Artem Oganov, Xiao Dong Gas hydrates are systems of prime importance. In particular, hydrogen hydrates are potential materials of icy satellites and comets, and may be used for hydrogen storage. We explore the H2O-H2 system at pressures in the range 0 $\sim$ 100 GPa with ab initio variable-composition evolutionary simulations. According to our calculation and previous experiments, the H2O-H2 system undergoes a series of transformations with pressure, and adopts the known open-network clathrate structures (sII, C0), dense ``filled ice'' structures (C1, C2) and two novel hydrogen hydrate phases. One of these structures is based on the hexagonal ice framework and has the same H2O:H2 ratio (2:1) as the C0 phase at low pressures and similar enthalpy (we name this phase Ih-C0). The other newly predicted hydrate phase has a 1:2 H2O:H2 ratio and structure based on cubic ice. This phase (which we name C3) is predicted to be thermodynamically stable above 38 GPa when including van der Waals interactions and zero-point vibrational energy. This is the hydrogen-richest hydrate and this phase has the highest gravimetric densities (18 wt.{\%}) of extractable hydrogen among all known materials. We thank the DARPA (Grants No. W31P4Q1310005 and No. W31P4Q1210008), National Science Founda- tion (EAR-1114313, DMR-1231586), AFOSR (FA9550- 13-C-0037), DOE (DE-AC02-98CH10886), CRDF Global (UKE2-7034-KV-11) for financial support. We thank Purdue University Teragrid for providing computational resources and technical support for this work (Charge No.: TG-DMR110058). [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F26.00007: High Pressure Study of Electrical Resistivity of CeB6 to 136 GPa Neda Forouzani, Jinhyuk Lim, James Schilling, Gilberto Fabbris, Zachary Fisk Since the 1960's the dense Kondo compound cerium hexaboride (CeB6) has attracted a great deal of interest. To investigate whether this material might evolve into a topological insulator under sufficient pressure, we have carried out four-point electrical resistivity measurements on CeB6 over the temperature range 1.3 K to 295 K in a diamond anvil cell to 136 GPa. Although a transition into an insulating phase is not observed, the evolution of the initial dense Kondo state under such extreme pressures is of considerable interest. As reported in earlier studies to 13 GPa [1], the temperature of the resistivity maximum near 3 K initially increases with pressure. We observe that between 33 and 53 GPa the resistivity maximum disappears and by 83 GPa CeB6 appears to have settled into a Fermi liquid state. The marked changes observed under pressure suggest that a change in valence and/or a structural transition may have occurred. Synchrotron x-ray diffraction measurements are being carried out to investigate possible changes in crystal structure under extreme pressures.\\[4pt] [1] Kobayashi et al., Physica B 281\&282, 553 (2000) [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F26.00008: Compression effects on electrons for chemical bonding Anguang Hu, Fan Zhang How electrons move under compression as chemical bonds between atoms are broken and formed is central to a number of challenges on the performance of materials in extreme conditions. This is not only associated with the fundamental knowledge of material response to compressive loading but also would advance many aspects of material science towards future energy technologies. First-principles simulations of enthalpy minimization, in various target pressures on chemical transformation bonding pathways, reveal that high pressure can push electrons away from their denser regimes where the kinetic energy rises steeply on compression, causing a destabilization of intramolecular bonds. The high-pressure pushing of electrons from one regime to another thus leads to chemical bond destruction and formation with a cell volume collapse accompanied by a drop in stress components. Determination of such electron pathways following bonding conformations of molecular precursors would then result in a number of chemical transformations for novel materials, including high energy density materials. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F26.00009: Elasticity of Fe- and Al-bearing and --free MgSiO$_{3}$-perovskite Gaurav Shukla, Zhongqing Wu, Renata Wentzcovitch We present a thorough analysis of the elastic properties of iron- and aluminum-bearing and --free MgSiO3-perovskite. Results from different first principles methods are compared to experimental data available and results for aggregate elastic moduli and velocities are analyzed at lower mantle conditions. Velocity heterogeneities produced by temperature variations and variation of aluminum and iron content are carefully examined and contrasted. This analysis is essential for improving understanding of the constitution of Earth's lower mantle. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F26.00010: Evolutionary Structure Prediction of Stoichiometric Compounds Qiang Zhu, Artem Oganov In general, for a given ionic compound A$_{m}$B$_{n\thinspace }$at ambient pressure condition, its stoichiometry reflects the valence state ratio between per chemical specie (i.e., the charges for each anion and cation). However, compounds under high pressure exhibit significantly behavior, compared to those analogs at ambient condition. Here we developed a method to solve the crystal structure prediction problem based on the evolutionary algorithms, which can predict both the stable compounds and their crystal structures at arbitrary P,T-conditions, given just the set of chemical elements. By applying this method to a wide range of binary ionic systems (Na-Cl, Mg-O, Xe-O, Cs-F, etc), we discovered a lot of compounds with brand new stoichimetries which can become thermodynamically stable. Further electronic structure analysis on these novel compounds indicates that several factors can contribute to this extraordinary phenomenon: (1) polyatomic anions; (2) free electron localization; (3) emergence of new valence states; (4) metallization. In particular, part of the results have been confirmed by experiment, which warrants that this approach can play a crucial role in new materials design under extreme pressure conditions. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F26.00011: Investigation of structural and magnetic properties of LaCo5 under pressure Markus Daene, Jason R. Jeffreys, Jon R.I. Lee, Daniel Aberg, Patrick Huang, Nick P. Butch, Scott K. McCall, Lorin X. Benedict, Babak Sadigh We report a joint experimental and theoretical investigation of the crystal structure parameters and magnetic moments of LaCo5 under hydrostatic compression. Theoretical predictions were made using density-functional-based electronic structure methods; special attention was paid to the dependence of the results on the choice of exchange-correlation functional. We comment on the degree to which our predictions match those of our measurements and relate both to earlier studies of Koudela et al. [1]. [1] D. Koudela et al., Phys. Rev. B vol.77, 024411 (2008). [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F26.00012: Hydrostatic High-Pressure Studies to 25 GPA on the Model Superconducting Pnictide LaRu2P2 Jinhyuk Lim, Neda Forouzani, James Schilling, Roxanna Fotovat, Chong Zheng, Roald Hoffmann Prior to the discovery of the Fe-pnictides in 2008, the ruthenium phosphide LaRu2P2 possessed the highest value of the su- perconducting transition temperature, Tc$\approx$ 4 K, in the entire pnictide family. Recently, there has been renewed interest in this compound in an effort to better understand why the Fe-pnictides have much higher values of Tc [1]. In related phosphides superconductivity appears to only be present if the separation be- tween the phosphor ions dp-p in neigh- boring Ru2P2 planes is greater than the critical value 2.8 {\AA}, too great for a P-P covalent bond to be formed. For example, in superconducting LaRu2P2, the value of dp-p is 3.0 {\AA}. To test these ideas directly, we have carried out hydro- static high-pressure studies on single-crystalline LaRu2P2 in a diamond-anvil cell using He pressure medium to pres- sures as high as 25 GPa and temperatures as low as 1.5 K. We find that Tc initially increases under pressure, but suddenly disappears above 2.1 GPa. Since dp-p decreases under pressure, the sudden disappearance of superconductivity is likely due to the formation of a covalent P-P bond between adjacent Ru2P2 planes and a possible structural phase transition.\\[4pt] [1] Razzoli et al., Phys. Rev. Lett. 108, 257005 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F26.00013: Hyperstoichiometric Oxygen in Fluorite-type U$_{3}$O$_{8}$ Formed at Extreme Conditions Fuxiang Zhang, Maik Lang, Rod Ewing U$_{3}$O$_{8}$ was obtained by annealing UO$_{3}$ in a reducing atmosphere at 200 $^{\circ}$C. Powder sample of $\beta $-U$_{3}$O$_{8}$ was pressurized at room temperature up to 37.5 GPa and XRD patterns clearly indicated that a phase transition occurred between 3-11 GPa. The high-pressure phase is a fluorite-like structure. The high-pressure phase was then laser heated to over 1700 K in the diamond anvil cell at high pressure conditions. No phase transition was found at high pressure/ temperature conditions, and the fluorite-like structure of U$_{3}$O$_{8}$ is even fully quenchable. The lattice parameter of the fluorite-like high-pressure phase is 5.425 {\AA} at ambient conditions, which is smaller than that of the stoichiometric UO$_{2}$. Previous experiments have shown that the stoichiometric uranium dioxide (UO$_{2})$ is not stable at high pressure conditions and starts to transform to a cotunnite structure at $\sim$ 30 GPa. When heating the sample at high pressure, the critical transition pressure is greatly reduced. However, the fluorite-like high-pressure phase of U$_{3}$O$_{8}$ is very stable at high pressure/high temperature conditions. The enhanced phase stability is believed to be related to the presence of extra oxygen (or U vacancies) in the structure. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F26.00014: Strong Pressure Dependence of Electrical Transport in V$_{2}$O$_{3}$ Thin Films Ilya Valmianski, Gabriel Ramirez, Siming Wang, Xavier Batlle, Ivan K. Schuller We present results of electrical transport measurements in V$_{2}$O$_{3}$ thin films under hydrostatic pressure from 100 KPa to 1.6 GPa. Uniaxial pressure and strain dependences of the metal-insulator transition temperature in V$_{2}$O$_{3}$ were extracted using a method previously established for high Tc superconductors [1]. Strain in the $z$ direction was calculated using V$_{2}$O$_{3}$ stiffness along the growth direction, while lateral strain was determined by the substrate properties. V$_{2}$O$_{3}$ thin films (100 nm) were grown epitaxially on three differently oriented single crystal Al$_{2}$O$_{3}$ substrates (a-plane, m-plane, and r-plane). Crystal phase purity and film quality were confirmed using high angle X-ray diffraction and X-ray reflectometry. All of the films showed a more than a four order of magnitude resistance change between the metallic and insulating states. The obtained pressure and strain dependences of the transition temperature may lead to novel device applications. \\[4pt] [1] S Bud'ko, J. Guimpel, O. Nakamura, M. Maple and I. K. Schuller, Phys. Rev. B, 1992, 46 1257 [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F26.00015: Comparative study of helimagnets MnSi and Cu$_2$OSeO$_3$ at high pressures Sergei Stishov, Vladimir Sidorov, Alla Petrova, Peter Berdonosov, Valery Dolgikh The heat capacity of helical magnets Cu$_2$OSeO$_3$ and MnSi has been investigated at high pressures by the ac-calorimetric technique. Despite the differing nature of their magnetic moments, Cu$_2$OSeO$_3$ and MnSi demonstrate a surprising similarity in behavior of their magnetic and thermodynamic properties at the phase transition. Two characteristic features of the heat capacity at the phase transitions of both substances (peak and shoulder) behave also in a similar way at high pressures if analyzed as a function of temperature. This probably implies that the longitudinal spin fluctuations typical of weak itinerant magnets like MnSi contribute little to the phase transition. The shoulders of the heat capacity curves shrink with decreasing temperature suggesting that they arise from classical fluctuations. In case of MnSi the sharp peak and shoulder at the heat capacity disappear simultaneously probably signifying the existence of a tricritical point and confirming the fluctuation nature of the first order phase transition in MnSi as well as in Cu$_2$OSeO$_3$. [Preview Abstract] |
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