Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V23: High Pressure VI |
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Sponsoring Units: DCMP Chair: Renata Wentzcovitch, University of Minnesota Room: Colorado Convention Center 110 |
Thursday, March 8, 2007 11:15AM - 11:27AM |
V23.00001: Potentially novel ultrahigh pressure form of ABX$_3$-type compounds Koichiro Umemoto, Renata Wentzcovitch By means of first-principles computations we have identified two new dynamically stable structures that are candidate ultra-high pressure forms of ABX$_3$-type compounds. To our knowledge, they have not been experimentally observed yet. They are produced by metastable pressure-induced transformations in $Cmcm$ NaMgF$_3$, a post-perovskite phase. The first transition to a $Pmcn$ structure is related to a soft phonon mode in post-perovskite. The second one is a regular enthalpically driven transition from $Pmcn$ to a $P6_3/mmc$ structure. In NaMgF$_3$ these phases are metastable with respect to the dissociation into CsCl-type NaF and cotunnite-type MgF$_2$. However, the $Pmcn$ phase might be observed at low temperatures. We have also identified a candidate post-perovskite material that prefers the $Pmcn$ phase over the dissociation into AX- and BX$_2$-type solids. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V23.00002: Reactive Molecular Dynamics Studies of Thermal Induced Chemistry in TATB Timothy Germann, Jason Quenneville Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was developed by van Duin, Goddard and coworkers [1] at CalTech and has already shown promise in predicting the chemistry in small samples of RDX under either shock compression or intense heat. Large-system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. We will show results of 800-particle simulations at several temperatures, and detail current capabilities for large-scale (10$^{4}$ -- 10$^{5}$ atoms) systems carried out with the massively parallel GRASP MD software developed at Sandia National Lab. Finally, we will compare the reaction timescales with those of RDX and HMX. [1] A. C. T. Van Duin, \textit{et al}, \textit{J. Phys. Chem. A}, \textbf{1005}, 9396 (2001). [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V23.00003: Long timescale dynamics of shocked nitromethane Laurence Fried, Evan Reed, Riad Manaa, Kurt Glaesemann, John Joannopoulos We present the farthest ever glimpse behind the shock front in a chemically reactive molecular dynamics simulation by applying a multi-scale shock wave simulation technique to the study of chemical reactions in shocked nitromethane (CH3NO2) represented by the DFTB tightbinding method. Shock speeds from 5.5 km/s to 8 km/s are simulated for durations up to 0.8 ns demonstrating substantial computational savings compared with the non-equilibrium molecular dynamics (NEMD)shock simulation approach. These simulations indicate that the reaction zone in detonating nitromethane is greater than 0.3 $\mu$m in length. Ionic species are found to be prevalent in the early reactions of shocked nitromethane. Results are consistent with available experimental data. As a validation of our multiscale approach, we compare spatial wave profiles computed with the multiscale technique to profiles computed using the NEMD approach. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V23.00004: Shear strain induced structural and electronic modifications of the energetic molecular crystal 1,1-diamino-2,2-dinitroethylene Sergey Rashkeev, Maija Kuklja First-principles calculations of the structural and electronic properties of the deformed energetic molecular crystal 1,1-diamino-2,2-dinitroethylene (FOX-7) under shear-strain loading are presented. The reaction of the crystal to applied shear-strain loading is found to be highly anisotropic. When the external loading is removed, the relaxation of the system is mainly defined by stretching, bending, and rotations of the NO$_{2}$ groups of neighboring molecules. In general, the deformed molecular crystal never relaxes to its initial, ideal crystalline FOX-7 structure. Instead, different planes remain shifted relatively to each other on vectors, which are typically incommensurated with any translational vector of the ideal crystal. We also found that no metallization occurs under shear-strain loading. We suggest that the considered mechanisms of the shear-strain relaxation of the structural and electronic degrees of freedom are typical for layered anisotropic molecular crystals, and that they should significantly affect their chemical reactivity, conductivity, optical properties, and initiation of detonation in energetic materials. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V23.00005: An Infrared Study of Secondary Explosives under High Pressure. Brian Yulga, Michael Pravica, Zhenxian Liu, Oliver Tschauner, Malcolm Nicol We report synchrotron FTIR and far infrared measurements on PETN, RDX, HMX and TATB at ambient temperature and high pressure, using various media for pressurization of the samples. In all cases, we have carefully studied any phase transitions in the 0 - 15GPa pressure range and have cycled pressures to interrogate sample survivability and reproducibility of the phase sequences. For PETN, we used differing pressurizing media (Ar and KBr) and have found that the onset of a prior-reported phase transition around 5GPa varies with the different media, portending the importance of sheer stress in inducing some or all of this phase transitions. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V23.00006: Vibrational spectra of solid HNFX (C$_6$H$_8$F$_8$N$_8$O$_4$): Experiments and theory Malcolm Nicol, Cedric Gobin, Eunja Kim Assignment of the vibrational spectra of molecular solids such as HNFX is very complex. We have made a combined experimental and modeling study of the vibrational spectra of solid HNFX. Crystalline HFNX consists of unit cell with 9 HNFX molecules in Ci symmetry. Vibrational modes were calculated by using the PCFF force field method and were directly compared to measured IR and Raman spectra. A complimentary calculation for molecular HNFX allows us to identify the intramolecular motions measured in experiments. Intermolecular motion by F--H bonds between HNFX molecules will be discussed in this talk. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V23.00007: High Pressure Behavior in Hydrated Metal Hexafluorosilicates M(H2O)6SiF6 Alice Acatrinei, Monika Hartl, Luke Daemen, Jianzhong Zhang, Yusheng Zhao The octahedral (6-fold) coordination is highly unusual in Si chemistry, making hexafluorosilicates (HFS) interesting from a structural standpoint. It has been observed that Si coordinates to O octahedrally at extremely high pressures deep in the Earth mantle, but no compound possessing this property is known to be thermodynamically stable at ambient conditions. We suggest that HFS could act as surrogate materials to study Si in this coordination state in hydrated materials. Transition metal HFS exhibit a variety of structural transitions and magnetic properties changes when pressure is applied. Some structural phases and phase transitions exhibited by some compounds at ambient pressure can occur in other compounds at high pressure only. We examined the behavior of Zn(II) as well as Cd(II) and Hg(II) HFS hexahydrates as a function of pressure and temperature, and their possible role in understanding structural phase transitions in HFS. Measurements were performed between 0-18 GPa at room temperature. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V23.00008: On the High-Pressure Behavior of Titanium Hydride Patricia E. Kalita, Stanislas Sinogeikin, Kristina E. Lipinska-Kalita, Thomas Hartmann, Andrew Cornelius Hydrogen storage research has recently invested a great deal of efforts into investigations of metal hydrides. Although titanium hydride is not the ideal candidate for storing hydrogen, Ti hydrides can act as active species to catalyze the reversible dehydrogenation of other hydrides and carbon nanotubes. In addition the basic science interest of this project lies in investigating the structure and especially the high-pressure behavior of TiH2. In the present study, we show the first in situ, high-pressure angle-dispersive and energy dispersive synchrotron x-ray diffraction studies of titanium hydride. We investigate the effects of hydrostatic and non-hydrostatic conditions. We also show the results of structural refinements as well as the bulk modulus of TiH2. To the best of our knowledge, this work is the first attempt to measure the equation of state of TiH2 using synchrotron x-ray diffraction and diamond anvil cells. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V23.00009: Pressure tuned phonon mode splitting in magnetic frustrated spinel ZnCr$_{2}$O$_{4}$ Tao Zhou, Zhenxian Liu, Chenglin Zhang, Sang-Wook Cheong ZnCr$_{2}$O$_{4}$ has cubic spinel structure. Below 390 K, the geometrically frustrated magnet enters a paramagnetic state. Below 12.5 K, it undergoes a first-order phase transitions, resulting into an antiferromagnetic order and a structural distortion simultaneously. An IR-active phonon related to the Cr$^{3+}$ ion's motion undergoes a splitting at 12.5 K. This transition is explained as a spin-Peierls like transition. However, the exact cause and effect in such a transition is not clear. Is it because the lattice undergoes transition first, spin just follows, or is it spins' interaction that forces the lattice to undergo changes? Pressure can provide a crucial service in clarifying this issue, since pressure can change spin and lattice interactions in different ways, it can differentiate these two scenarios. We have measured the infrared absorption spectra of ZnCr$_{2}$O$_{4}$ under pressure. Our data shows that Tc, at which the spin-Peiers like transition occurs and the phonon at about 370 cm$^{-1}$ starts to show the splitting, increases from its ambient pressure value of 12.5 K to about 15.8 K at 1 GPa. This provides an important clue for the exact nature of this transition. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V23.00010: Optical properties of CdSe semiconductor nanocrystals under high pressures Chi-Tsu Yuan, Wu-Ching Chou, Der-San Chuu In general, the physical properties of semiconductor nanocrystals are different from the bulk materials. CdSe nanoparticles are attracted much attention due to excellent fluorescence properties for potential applications in biological labels$^{1}$. In particular, the emission colors can be tuned to cover whole visible range by changing particle size with the same chemical composition. On the other hand, high pressure technique is another tool to tune the electronic states of crystalline materials. Incorporated colloidal QDs under high pressure environment can provide valuable information to study the electronic and vibrational states of nanometer size materials. In this study, the electronic and vibrational states of colloidal core/shell CdSe/ZnS quantum dots are studied at room temperatures by using high pressure optical measurements. Pressure dependent quadratic lattice behavior can be observed clearly from photoluminescence (PL) and Raman spectra up to $\sim $7 GPa. This quadratic relationship is consistent with theoretical prediction. The average pressure coefficients for PL and Raman measurements, as well as deformation potential are 32 meV/GPa, 4.2 cm$^{-1/}$GPa and -1.69 eV, respectively. [1] M. Jr Bruchez, M. Moronne, P. Gin, S. Weiss, and A. P. Alivisatos, Science \textbf{281}, 2013 (1998). [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V23.00011: High Pressure Characterization of the thermoelectric Bi2Te3 Matthew Jacobsen, Ravhi Kumar, Andrew Cornelius One of the current goals of the US Department of Energy is to find a new energy source that is non-reliant on fossil fuels for the production of energy. In an effort to address this concern, we have developed facilities to investigate the fundamental properties of thermoelectric materials utilizing the technique of pressure tuning. Pressure tuning, or application of high pressures, causes a material to undergo distinct, but controllable changes, to the physical properties. To this end, results of reference standards will be presented along with the first comprehensive set of data on the thermoelectric materials Bi2Te3. [1]Chen, G. International Materials Reviews, \textbf{48}, 45-66 (2003). [2]Jacobsen, M.K., Masters Thesis, in preparation, (2006). [3]Khvostansev, L.G. Phys.Stat.Solids A, \textbf{71}, 49-53 (1982). [4]Venkatasubramanian, R. Nature, \textbf{413}, 597-602, 2001. [5]Vereshchagin, L.F. et.al. Soviet Physics-Solid State, \textbf{13}, 2051-2053 (1972). [6]Yamashita, O. J.Mat.Sci.,textbf{40},6439-6444 (2005). [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V23.00012: Pressure dependance of the Curie temperature of TbNi$_2$Mn, investigated using designer diamond anvils Damon Jackson, Scott McCall, Samuel Weir, Dave Young, Qiu Wei, Yogesh Vohra \\ TbNi$_2$Mn is a cubic Laves structured material with a Curie temperature at ambient pressure of $T_C=151$~K. The behavior of the Curie temperature has been investigated by AC magnetic susceptibility under both hydrostatic and non-hydrostatic conditions using designer diamonds up to 29~GPa, for which it was found to decrease at $dT_C/dP$=-2.0~K GPa$^{-1}$. However, non-hydrostatic conditions result in a flattening out of $T_C$ with pressure which is non-reservable. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V23.00013: Spectroscopic evidence for pressure-induced metallization in solid silane Xiao-Jia Chen, Viktor V. Struzhkin, Alexander Goncharov, Yang Song, Zhen-Xian Liu, Ho-kwang Mao, Russell J. Hemley Infrared reflectance measurements on solid silane SiH$_{4}$ have been performed under pressure up to 70 GPa at room temperature. After passing through three phase transformations, solid SiH$_{4}$ is already black at 30 GPa. At high pressures around 60 GPa, the infrared reflectance spectra exhibit a Drude metallic behavior, signaling the pressure-induced metallization in solid silane. Angle-dispersive powder x-ray diffraction studies reveal that a structural transition is accompanying the silane transition to the metallic state. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V23.00014: Search for superconductivity in LiBC at high pressure Amy Lazicki, Choong-Shik Yoo, Hyunchae Cynn, William J. Evans, Warren E. Pickett, Justin Olamit, Kai Liu, Y. Ohishi Lithium borocarbide, which is a structural and electrical analog to high-T$_c$ superconductor MgB$_2$, remains insulating at ambient conditions due to atomic alternation in the crystal structure. We investigated experimentally and theoretically the properties of this material under pressure, including structural and bonding anisotropy and the possibility of metallization and superconductivity under high pressure. It is found to remain stable up to 60 GPa with no crystal structure change and without a previously reported lattice parameter anomaly. In this crystal structure, metallization is not predicted to occur until at least 345 GPa, at which pressure the electronic bands responsible for superconductivity in MgB$_2 $ remain unoccupied in LiBC, ruling out the possibility of a new MgB$_2$-like high pressure superconductor. [Preview Abstract] |
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