Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F39: Focus Session: Materials in Extremes: High Pressures |
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Sponsoring Units: GSCCM DCOMP DMP Chair: Evan Reed, Stanford University Room: 348 |
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F39.00001: Pressure-induced metallization and phase transitions in GeS$_2$ Ranga Dias, Choong-Shik Yoo We have studied the pressure-induced structural and electronic phase transitions of crystalline GeS$_2$ ($P$2$_{1}/c)$ to 50 GPa, using micro-Raman spectroscopy and electrical resistivity measurements in diamond anvil cells. The result shows a steady decrease in resistivity to that a metal at around 40GPa. The visual appearance of GeS$_2$ supports the insulator-metal transition: initially transparent GeS$_2$ becomes opaque and eventually reflective with increasing pressure. The Raman and X-ray diffraction result indicates that the metallization is preceded by a structural phase transition. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F39.00002: Density Functional Theory Investigation of Sodium Azide at High Pressure Brad Steele, Aaron Landerville, Ivan Oleynik Sodium azide is intriguing because it could potentially be used as a precursor to a high-nitrogen energetic material. Furthermore, recent absorption and Raman spectroscopic results have shown that novel nitrogen structures may indeed be attainable from sodium azide. First-principles density functional theory calculations were performed to characterize possible novel crystalline structures of sodium azide including their atomic structure, vibrational properties, Raman spectra, and equation of state up to 90 GPa. Calculated Raman peaks and intensities show good agreement with experiment. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F39.00003: Simple binary mixtures of hydrogen and ammonia under extreme pressures Gustav Borstad, Choong-Shik Yoo Binary mixtures under pressure are of interest as fundamental systems in physics and chemistry as they allow the effects of the environment on the behavior of different chemical compounds to be examined. Furthermore, mixtures of simple molecular systems are of interest for applications in fuel cells and also to planetary science due to their presence in the interiors of the giant gas planets. In this presentation, Raman data on the ammonia and hydrogen system under pressure will be presented, and the extent and nature of the interactions in this mixture will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F39.00004: First-principles simulations on chemical transformation bonding pathways of compressed graphite Anguang Hu, Fan Zhang Chemical transformation bonding pathways for cubic diamond, hexagonal diamond, and cold compressed carbon has been investigated using first-principles simulations of the enthalpy minimization with various target pressures. The high-pressure bonding pathways of carbon from initial bonding conformations can be divided into three bonding evolution stages, which are defined as the van der Waals bonding destruction, bond breaking and forming reaction, and bonding equilibrium process. The principal stress tensor components were used to characterize the response of C-C bonds in graphite to compressive loading. It was found that the local stress field starts to rapidly increase towards the positive direction at the onset of the van der Waals bonding destruction. Bond breaking and forming reaction then takes place, leading to a cell volume collapse accompanied with a drop in stress components. The three bonding evolution stages demonstrated that the bonding evolution of the system towards chemical transformation under compression can be dictated by the local stress field together with the initial bonding conformation. Thus, the local stress field provides an understanding on how atoms and electrons move during the course of chemical transformation under compression. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F39.00005: Thermo-Physical Properties of Ammonium Azide under High Pressure from First-Principles Aaron Landerville, Brad Steele, Ivan Oleynik Polynitrogen compounds offer tremendous promise for use as insensitive high-explosives or propellants. While the existence of such compounds have been observed in Diamond Anvil Cells (DAC) under high pressure, recovery to ambient pressure and temperature has proven problematic. A current thrust towards the recovery, and ultimate manufacture, of materials rich in polymeric nitrogen has brought renewed attention to various nitrogen-rich compounds, particularly crystalline azides, as possible precursors. We investigate the thermo-physical properties and Raman spectra of one azide candidate -- ammonium azide -- under hydrostatic compression using density functional theory with an empirical van der Waals correction. Additionally, we perform structural minima searches to discern possible polymorphs that may help to elucidate dynamical processes leading to the production of a material rich in polymeric nitrogen, as well as its recovery from DAC. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F39.00006: Theory-driven discovery of an exotic CaB$_{6}$ high-pressure crystal structure phase Aleksey Kolmogorov, Sheena Shah, Elena Margine, Annette Kleppe, Andrew Jephcoat We synthesized and solved an unexpectedly complex crystal structure of CaB6 under high pressures and temperatures [1]. The only known crystal structure in the large family of metal hexaborides, a simple cubic cP7 type, has been shown to transform into a tetragonal tI56 configuration comprised of unfamiliar 24-atom boron units above 30 GPa and remain metastable under ambient pressure. The interpretation of the convoluted x-ray diffraction pattern was accomplished with an ab initio evolutionary search implemented in MAISE [2] which identified the tI56 structure (28 atoms per primitive unit cell) without any parameter input, i.e. truly ``from scratch.'' I will describe the performance of different ground state search techniques in such challenging cases.\\[4pt] [1] A.N. Kolmogorov et al., Phys. Rev. Lett. 109, 075501 (2012)\\[0pt] [2] Module for Ab Initio Structure Evolution, http://maise-guide.org [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F39.00007: Modeling of the amorphous phase of poly-CO I.G. Batyrev We studied theoretically the details of amorphous structure of extended CO solid obtained by isotropic compression of solid CO phases in the range of 3-25 GPa. We performed DFT simulations of 128, 432, and 1024 atom models. Structures of random networks found at zero temperature were used for equilibration at finite temperatures up to 50 ps by employing first principles MD. We found that the polymerization begins at 6 - 8 GPa and a random network of 4-7 atom rings obtained above 15 GPa could exist up to 0.1 -0.25 GPa. We studied pressure induced changes in topological characteristic of the random network based on the rings statistics, radial distribution function and average number of the nearest neighbors (NN). NN found to be 3.2 for C and 1.7 for O for 128 atom system at 15 GPa. We performed vibration analysis of the systems as a function of pressure and calculated in dipole approximation IR intensity with identification of contributions from several main motifs of the amorphous structure. To understand charge distribution and localization and to find the possible ``weakest link'' in the network we calculated electron localization function for the most common fragments of amorphous poly-CO structure. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F39.00008: The role of anharmonicity in the ab-initio phase diagram of calcium Marco Di Gennaro, Srijan Kumar Saha, Matthieu Jean Verstraete In the 32-119 GPa pressure range and at room temperature, a simple cubic phase was reported for calcium in many different experiments. Standard linear response theory, both within density functional perturbation theory and frozen phonon calculations, presents dynamical instabilities for simple cubic in the whole pressure range. Many other possible candidate phases, as well as several possible stabilization mechanisms for simple cubic phase, have been proposed as the result of \emph{ab-initio} predictions but the role of temperature on the relative stability of the different phases has not been investigated systematically. We revisit the stability of three candidate phases of calcium for the intermediate pressure range and for various value of temperatures, taking explicitly into account thermal corrections relative to electronic as well as phononic entropy and anharmonic contributions. This corrects the discrepancies among previous theoretical results and experiments, and presents a different picture of the temperature driven phase transition, which results from dynamical anharmonic stabilization of simple cubic and de-stabilization of the tetragonal phase. Transport quantities are calculated in the stabilized phases, to provide additional points of comparison with experiments. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F39.00009: Origin of Metallization of FeO at High Temperatures and Pressures from First-principles DFT-DMFT Computations R.E. Cohen, Kristjan Haule Experiments and theory show that FeO metallizes at high temperatures ($\sim$2000K) and pressures ($\sim$80 GPa) [1]. Here we use DFT+Dynamical Mean Field Theory (DMFT) with continuous time quantum Monte Carlo (CTQMC) to study the origin of the metallization. We find with increasing pressure in paramagnetic FeO in a cubic lattice a high-spin low-spin transition, with a wide transition region between characterized by intermediate occupancies of the t2g and eg states between. We find that at 300K cubic FeO remains insulating to a factor of two compression (over 600 GPa), except for a small region of high spin metal. However, at high temperatures (e.g. 2000K) a metallic state is found under compression. The metallization occurs from thermal fluctuations among different multiplets representing high- and low-spin states. We are now studying the AFM ground state, the N\'eel transition, and (Mg,Fe)O solid solutions. This work is supposed by NSF.\\[4pt] [1] Ohta, K., Cohen, R. E., Hirose, K., Haule, K., Shimizu, K. \& Ohishi, Y. Experimental and Theoretical Evidence for Pressure-Induced Metallization in FeO with Rocksalt-Type Structure. Phys. Rev. Lett. 108, 026403 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F39.00010: Towards a Predictive First-Principles Description of High Pressure Hydrogen with Density Functional Theory Miguel A. Morales, Jeffrey M. McMahon, Carlo Pierleoni, David M. Ceperley We present a study of the influence of the main approximations employed in first-principles descriptions of high pressure hydrogen with Density Functional Theory. We focus on the importance of nuclear quantum effects (NQE) on equilibrium properties of both liquid and solid molecular hydrogen close to dissociation. We find that NQEs strongly influence intramolecular properties, such as bond stability, and are thus an essential part of the dissociation process. In addition, we show how the combination of both thermal and quantum effects make a drastic change to the predicted optical properties of the molecular solid, demonstrating the very limited value of predictions based on classical ions and static crystals. We also focus on the influence of the chosen exchange--correlation density functional on the predicted properties of hydrogen, including the location of the Liquid-Liquid Phase Transition and the pressure dependence of the band gap in the solid. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F39.00011: Lattice dynamics beyond the harmonic approximation: a compressive sensing approach Fei Zhou, Weston Nielson, Vidvuds Ozolins First-principles modeling of materials in extreme conditions of increasing complexity has had profound impact on revealing and predicting the materials properties and explaining experimental results. Therefore methods and algorithms that can automatically scale to large systems with quantum mechanical accuracy are in dire need. Recently we have shown that a recently developed technique in the field of signal processing, compressed sensing (CS), provides a simple, general, and efficient way of constructing cluster expansion models for alloy systems. Here CS is applied to calculate force constants, including anharmonic effects up to high orders, in solids. CS performs well in extracting accurate lattice dynamics with highly competitive computational costs and reduced human efforts. Compressive sensing for lattice dynamics can be readily applied to much larger systems than ab initio methods can handle and with superior accuracy than classical force fields. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F39.00012: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F39.00013: Neon Hydrate at High Pressure: an in-situ Neutron Diffraction Study Xiaohui Yu Clathrate hydrates are a group of ice-like, crystalline inclusion compounds which form through the combination of water and suitably sized ``guest'' molecules. There are mainly three crystallographic structures of the hydrate clathrate: SI, SII and SH, which are determined by the shape and size of the included gas molecular. However, when the neon gas pressure got increased to 0.48 GPa, we found that the neon gas could be enclathrate in the ice II frameworks which is totally different structure from the traditional cubic clathrate. Through the in-situ neutron diffraction study, the detail structure of Ne hydrate, including the atom positions, can be derived using the Rietveld refinements. The Ne atoms are just in the middle of H$_{2}$O channels and sandwiches by two H$_{2}$O rings The thermal equation of state was calculated and compared with pure ice II. We found that inclusion of Ne atoms could enlarge the ice II H$_{2}$O hexagonal rings, however, shortened the H$_{2}$O channels. Although the Ne atoms crystallized in ice II frameworks, the thermal vibration is significant compared to the host atoms. The distribution of Ne atoms are presented by MD simulations. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F39.00014: Neutron Scattering Study of Hydrogen in Copper Alexander I. Kolesnikov, Vladimir E. Antonov, Garrett E. Granroth, Valery I. Kulakov, Michail A. Kuzovnikov, Ken C. Littrell, Eugene Mamontov Until now, vibrational spectra of hydrogen in the group 1b metals, Cu, Ag and Au, have never been investigated. Meanwhile, these elements are often used in hydrogen containing atmospheres, therefore the properties of hydrogen in these metals are of significant interest. For the present study, Cu-H samples were synthesized by exposing bulk copper to a hydrogen gas at a pressure of 7 GPa and T=900 K and recovering the samples to ambient conditions. The samples were studied by inelastic (INS), quasielastic and small angle neutron scattering. Nearly all hydrogen ($\sim$10 at.\%) contained in the samples proved to be in the form of H2 molecules trapped in large ($>>$100 A) pores/bubbles in the copper matrix. Para $<=>$ ortho transitions in these molecules give intense peaks at +-14.4 and 28.8 meV in the INS spectra. On heating the sample, the molecular hydrogen melts in a temperature interval from 14 to 60 K corresponding to the gradual increase in the H2 pressure in the pores from 5 bar to 3 kbar. A small narrow peak at 73 meV is also observed in the INS spectra. The peak can only be assigned to a local mode of a 0.03 at.\% H impurity in the copper bulk. This is the first observation of H vibrations in a group 1b metal. [Preview Abstract] |
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