Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H11: Focus Session: Simulations of Matter at Extreme Conditions II |
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Sponsoring Units: DCOMP DMP GSCCM Chair: Stephane Mazevet, LANL Room: LACC 153C |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H11.00001: Melting and metallization of compressed hydrogen: Predictions from first-principles calculations Invited Speaker: A long-sought goal in condensed matter physics has been the metallization of compressed hydrogen. So far, it has been achieved only in the fluid phase at high temperature. In this talk, I will discuss recent results predicting the melting line and a first-order liquid-liquid phase transition from molecular to dissociating fluid in compressed hydrogen [1]. The phase boundaries are computed from first principles using molecular dynamics. The results indicate that there is a maximum in the melting curve of hydrogen and open up the interesting possibility of finding a low-temperature metallic fluid at pressure around 400 GPa. The existence of a maximum melting temperature is rather unusual for a close-packed solid structure. Our analysis indicates that in hydrogen it is a result of changes in intermolecular interactions, occurring in the fluid phase at high pressure. S. A. Bonev, E. Schwegler, T. Ogitsu and G. Galli, \textit{Nature} \textbf{431}, 669-672 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H11.00002: Path Integral Simulations of Hydrogen Melting at High Pressures Rebekah Graham, Burkhard Militzer The melting line of hydrogen at high pressure and low temperature is studied using computer simulations.~Using path integral Monte Carlo we focus on the regime where the protons form a Wigner crystal. The Lindemann ratios, structure factors, and pair correlation functions are used to characterize melting due to increases in both pressure and temperature. The effects of electron screening on the stability of the crystal are investigated by comparing results from Coulomb and Yukawa simulations. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H11.00003: Hydrogen Bonding at Extreme Conditions Riad Manaa Triamino-trinitro-benzene (TATB) exhibits unusually strong intramolecular hydrogen bonding as evidenced by the high rotational energy barrier of the nitro and amino groups. In the condensed phase, competing intermolecular hydrogen bonding becomes pronounced at high-pressure. In this talk, I will present the results of recent first-principal computational studies concerning the effect of isotropic high-pressure on the atomistic structure of TATB up to 250 GPa. Along a specific crystal lattice direction, there exists a pressure regime beyond which both the inter and intra-molecular hydrogen bonding become equivalent. In the lower pressure regime of up to 50 GPa, the prediction of theory will be compared with on going experimental high-pressure X-ray diffraction studies. Possible mechanisms of phase transitions in the lower pressure regime will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H11.00004: Incommensurately modulated sulfur at megabar pressures: experimental and computational study Olga Degtyareva, Razvan Caracas, Eugene Gregoryanz, Ronald Cohen, Russell Hemley Recent discoveries of incommensurate host-guest and/or modulated phases in elemental metals at high pressure suggest that aperiodic structures are a common phenomenon among the elements under pressure. However, the driving force for development of the incommensurability and structural modulations in these elemental systems is poorly understood. Using synchrotron x-ray diffraction, we show that the metallic phase of sulfur, stable above 86 GPa, has an incommensurately modulated (IC) crystal structure similar to those reported for Te and Se [1], with a strong pressure dependence of the modulation vector. The IC phase transforms to a $\beta$-Po rhombohedral structure at 140 GPa. Theoretical calculations show a further transition to a bcc phase. We investigate by first-principles calculations the possibility of an electronic instability at the origin of the incommensurability. We analyze the electronic band structure of the average crystal structure and of several commensurate approximants and examine the development of nesting in the Fermi surface. Our calculations also show the development of hybridization between the s and p states of S with increasing pressure. [1] C. Hejny and M.I. McMahon, PRL 91, 215502 (2003). This research is supported by DOE-NNSA (CDAC), DOE-BES, NSF and Carnegie Institution of Washington. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H11.00005: Time scale for rapid resolidification in the presence of competing solid phases Fred H. Streitz, Mehul V. Patel, James N. Glosli \noindent We investigate the time scale for pressure-induced solidification in a molten metal by simulating the process using molecular dynamics techniques. We find that the time to solidification in the simulation depends on the availability (or lack thereof) of competing solid phases into which the liquid can crystallize. We demonstrate this dependence using both a highly accurate, quantum-based potential (the MGPT potential) and simple, Morse-like potentials that have been modified to alter the relative stability of various solid phases. \vskip 1.5em \noindent Work performed under the auspices of the U.S. DOE at the University of California/Lawrence Livermore National Laboratory under contract W-7405-ENG-48 [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H11.00006: Rapid pressure-induced solidification of molten metals Mehul Patel, Frederick Streitz The process in which a molten metal subjected to sudden pressurization transforms into a solid is studied by large scale molecular dynamics simulations. Specifically, copper and tantalum are modeled as prototypical fcc and bcc metals. Copper interactions are described by an EAM potential, while tantalum is described with a more sophisticated MGPT potential including up to 4-body interactions. Questions about the structure of the final solidified state as well as the mechanisms (ie, metastable intermediaries) involved in getting there are answered by these atomistic simulations, and recent algorithmic and computational advances have allowed for studies of increasing system sizes and simulation times. In particular, the effects of varying physical parameters such as the magnitude and rate of pressurization on the total solidification time will be discussed. \vskip 0.2in \noindent Work performed under the auspices of the U.S. DOE at the University of California/Lawrence Livermore National Laboratory under contract W-7405-ENG-48 \\ [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H11.00007: Beryllium at high pressure and temperature: A first-principles molecular dynamics study Andrea Trave, Eric Schwegler, Francois Gygi, Giulia Galli \textit{Ab initio} simulations of metals under extreme pressure and temperature conditions are very challenging from a computational standpoint, compared, e.g., to simulations of semiconductors or insulators. Here we present large scale \textit{ab initio} molecular dynamics simulations of a compressed, simple metal at high temperature, beryllium; our calculations are carried out using a two-phase formalism, along with efficient algorithms to determine the electronic ground state at finite temperature, at each ionic step. Our results for the equation of state at 0 K show excellent agreement with previous experiments, indicating that Density Functional Theory can accurately describe the properties of beryllium in the ground state. Melting temperatures as a function of pressure and Hugoniot curves determined from first principles will be presented, and recent shock melting experiments will be interpreted and discussed. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H11.00008: Covalency in the superionic phase of water Laurence Fried, Nir Goldman We detail herein results of \textit{ab initio} Molecular Dynamics simulations of water at temperatures of 1000 -- 2000K, and densities of 1.8 -- 3.0 g/cc. We have calculated the lifetimes and concentrations of molecular and non-molecular species, and ionic conductivity and vibrational spectra. Comparison is made to experiment where possible. We observe the onset of a superionic phase at much lower temperature and pressure than previously calculated. Results indicate that at these conditions, water undergoes several transformations in which at higher densities, the oxygen atoms form a glassy state, and the hydrogens diffuse extremely rapidly by jumping between oxygen ``lattice'' points. We also find that at the superionic phase transition, molecular species are too short lived to be described as molecules or ionic conductors, and are better described as ensembles of transitions states. We argue that water in the so-called superionic phase is best described as consisting of extensive transient networks of O---H bonds, which are predominantly covalent. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H11.00009: Multiscale Modeling of the Onset of Void Coalescence in Dynamic Fracture Robert E. Rudd, Eira T. Seppala, James Belak In dynamic fracture of ductile metals, voids nucleate, grow and coalesce to form the fracture. Previously, we have studied the nucleation and growth process for voids at the nanoscale in a variety of FCC and BCC metals. Here we investigate the coalescence process. Coalescence is important because it initiates accelerated void growth leading rapidly to failure. Using large-scale parallel molecular dynamics (MD) simulations, we have characterized the coalescence through the void volume and shape, the concomitant dislocation activity, and the rate of impingement of the void surfaces. We find that the onset of void coalescence takes place when the separation between the voids, the inter-void ligament distance, is equal to one void radius. We compare the material flow in the MD simulations to that in continuum models of void coalescence. $^1$ E. T. Sepp\"al\"a, J. Belak, and R. E. Rudd, to appear in Phys Rev Lett (2004). Acknowledgment: This work was performed under the auspices of the US Dept. of Energy at the Univ. of California/Lawrence Livermore National Laboratory under contract W-7405-Eng-48. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H11.00010: Atomistic-based Dislocation Mobility Model for Material Strength at High Pressure Lin H. Yang, John A. Moriarty Understanding and predicting the mechanical behavior of materials at a range of temperatures, pressures, and strain rates require a detailed knowledge of the underlying mechanisms that govern plasticity. In this work, we focus on the high-pressure plastic deformation properties of bcc metals in which the plasticity is controlled by $a/2<111>$ screw-dislocation behavior in the crystalline lattice. In particular, the finite-temperature motion of the screw dislocation is believed to be associated with the formation of mobile kinks on the screw dislocation line. Thus, the accurate prediction of kink-pair activation energetics is essential to the understanding and determination of the mobility of screw dislocations in these materials. In turn, an atomistic-based dislocation mobility model for $a/2<111>$ screw dislocation is a key ingredient needed to develop predictive multiscale simulations of crystal plasticity for bcc metals. Based on the data obtained from atomistic simulations, we have developed a scaling relation and strength model for bcc metals under extreme conditions. The predicted flow stress as a function of temperature, strain rate and pressure for bcc Ta and Mo will be presented. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H11.00011: Deformation near grain boundaries in rocksalt structured ceramics at high strain rates James Palko, John Kieffer The rocksalt structure is one of the most basic ceramic crystal structures, and provides a useful model to study the deformation processes that occur near grain boundaries in ceramic materials. We used molecular dynamics and electronic structure calculations to explore the structure and deformation modes of high angle tilt grain boundaries in these materials, particularly NaCl, under various stress states at high strain rates. These simulations show the effects of dislocation impingement and emission on the structure of these grain boundaries, and the nucleation of cracks via the Zener-Stroh process. Lower dimensional structures are found to play a role in strength retention during fracture and grain boundary sliding processes. The stability of these structures was verified using ab-initio electronic structure calculations. Finally, an expanded hexagonal phase was discovered as a unique stress relief mechanism for a particular grain boundary structure. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H11.00012: Absence of B1-B2 structural transition in lithium halides under hydrostatic pressure Romeo de Coss, Gabriel Murrieta We have investigated the B1-B2 structural transition in LiF, LiCl, LiBr, and LiI under hydrostatic pressure by means of first-principles total-energy calculations using the Full- Potential LAPW method. In order to analyze the gradient effects, we have performed calculations using the local density approximation (LDA) and the generalized gradient approximation (GGA), for the exchange and correlation potential. In agreement with the experimental observations, we find that even for pressures higher than 100 GPa, the Li halides do not present the B1-B2 structural transition. In order to understand this behavior, we have calculated the distribution of the electron densities. From the analysis of the distribution of electron densities for the Li halides in the B1 and B2 phases, we find that for this group of ionic compounds the B1 phase have a distribution of electron densities more homogeneous than in the B2 phase, preventing the B1-B2 structural transition. This work was partially supported by Consejo Nacional de Ciencia y Tecnolog\'{\i}a (CONACYT, M{\'e}xico) under Grant No. 43830-F. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H11.00013: Anharmonic Materials and Thermoelasticity at High Temperatures and Pressures Daniel Orlikowski, Randolf Q. Hood, Per Soderlind, John A. Moriarty For large-scale constitutive strength models, the shear modulus is typically assumed to be linearly dependent on temperature. However, for materials compressed along or beyond the Hugoniot into high pressure and temperature regimes where there is no experimental measurement or very little, accurate and validated models must be used. To this end, we have investigated and compared, as a function of temperature ($<$26,000 K) and pressure ($<$10 Mbar), the anharmonic and quasi-harmonic thermoelasticity accounting for both the electron-thermal and ion-thermal contributions for bcc tantalum and bcc molybdenum. In this approach, the full potential linear muffin-tin orbital (FP-LMTO) method for the cold and electron-thermal contributions is closely coupled with ion-thermal contributions. For the ion contribution two separate approaches are used. In one approach, the quasi-harmonic ion contribution is obtained through a Brillouin zone sum of strain derivatives of the phonons, and in the other the anharmonic ion contribution is obtained directly through Monte Carlo (MC) canonical distribution averages of strain derivatives on the multi-ion potential itself. Both methods for the ion-contribution use quantum-based interatomic potentials derived from model generalized pseudopotential theory (MGPT). The resulting elastic moduli are compared to available ultrasonic measurements and diamond-anvil-cell compression experiments, as well as to sound speeds along the Hugoniot. Over this range of temperature and pressure, the results are then used in a polycrystalline averaging for a comparison to larger-scale shear models like the Steinberg-Guinan strength model. These results give an indication that anharmonic effects are negligible in tantalum but not in molybdenum for high pressures and temperatures up to melt. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract W-7405-Eng-48. [Preview Abstract] |
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