Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session C3: TM First Principles Methods II |
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Chair: Joel Kress, Los Alamos National Laboratory Room: Fifth Avenue |
Monday, July 8, 2013 11:00AM - 11:30AM |
C3.00001: \textit{Ab initio} molecular dynamics study of pressure-induced amorphization in sulfur Invited Speaker: Roman Marto\v{n}\'ak We report results of \textit{ab initio}constant-pressure molecular dynamics simulations of sulfur compression leading to structural transition and pressure-induced amorphization [1]. Starting from the orthorhombic S-I phase composed of ring molecules we find at room temperature and pressure of 20 GPa a transformation to monoclinic phase where half of the molecules develop a different conformation. Upon further compression, the monoclinic phase undergoes pressure-induced amorphization into an amorphous phase, in agreement with experiments [2,3]. We further study the dynamics of the amorphization transition and focus on the evolution of intra and intermolecular distances in the monoclinic phase in order to provide a microscopic insight into the rings disintegration process leading to amorphization. In the amorphous form we examine the structural properties and discuss its relation to the experimentally found amorphous form as well as to the underlying crystal phases. The amorphous form we find appears to correspond to the experimentally observed low density amorphous form [3]. \\[4pt] [1] Du\v{s}an Pla\v{s}ienka and Roman Marto\v{n}\'{a}k, Phys. Rev. B, 094112 (2012)\\[0pt] [2] H. Luo and A. L. Ruoff, Phys. Rev. B , 569 (1993)\\[0pt] [3] C. Sanloup, E. Gregoryanz, O. Degtyareva, and M. Hanfland, Phys. Rev. Lett., 075701 (2008) [Preview Abstract] |
Monday, July 8, 2013 11:30AM - 11:45AM |
C3.00002: Crystal Structure Searching by Free Energy Surface Trekking: Application to Carbon above 1 TPa Takahiro Ishikawa, Naoshi Suzuki, Katsuya Shimizu Crystal structure determination of materials under extreme conditions has been one of grand challenges in high-pressure materials science. In computer simulations, the crystal structure searching is carried out by exploring Gibbs free energy surface (GFES) at given pressures and temperatures. Here, we propose a new crystal structure searching technique named as free energy surface trekking (FEST). FEST is based on a very simple idea and consists of an ascent-run and a descent-run. In the ascent-run, the system is forced to ascend GFES from a starting local minimum by following the inversion of the driving force acting on the simulation cell. Then, the system descends it toward a neighboring local minimum by flipping the inverted force at the ridge of GFES. The details of GFES around the starting local minimum are more correctly obtained by more investigating different trekking routes. We have applied FEST to carbon at 1.2 TPa and at 300 K, and successfully obtained the transition from the cubic diamond phase to the previously predicted BC8 phase. In this transition, 3 cell-angles concurrently increase from 90$^{\circ}$ to 101$^{\circ}$ in the ascent-run and become 109$^{\circ}$ through the descent-run, in which the activation energy is approximately 0.17 Ry/atom. [Preview Abstract] |
Monday, July 8, 2013 11:45AM - 12:00PM |
C3.00003: Theoretical and Experimental Study of A$_3$B$_5$O$_{12}$ Garnets Under High Pressure Alfonso Munoz, Virginia Monteseguro, Placida Rodriguez-Hernandez, Francisco Javier Manjon, Victor Lavin In the last years oxide garnets are being used for technological applications in the field of solid state materials, especially as active matrices for lasers. Features such as high thermal conductivity, hardness, and chemical and mechanical stability make them good host matrices for luminescent Rare Earth (RE$^{3+})$ ions. In this sense, large efforts have been spent to investigate the luminescence properties of (RE$^{3+})$ doped nano-structured garnets , especially in the development of lasers and phosphors in lightning applications and as an alternative to quantum dots in the development of photonic and optoelectronic devices. In this contribution we will present a combined \textit{ab initio} and experimental study of the structural, dynamical and mechanical stability properties of some A$_{3}$B$_{5}$O$_{12}$ garnets under high pressure. [Preview Abstract] |
Monday, July 8, 2013 12:00PM - 12:15PM |
C3.00004: 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] |
Monday, July 8, 2013 12:15PM - 12:30PM |
C3.00005: 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] |
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