Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q13: Focus Session: Extreme Conditions and High Pressure II: Phase Transitions |
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Sponsoring Units: DCOMP GSCCM Chair: Amy Lazicki, Geophysical Laboratory Room: 309 |
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q13.00001: Computation of the Iron and Iron-Nickel Phase Diagrams from Ambient to Earth's Inner Core Xuan Luo, R.E. Cohen We have performed first-principles computation for the magnetic iron and iron-nickel over a wide range of pressure (0-400GPa) and temperature (0-6000K), within density-functional theory (DFT) in the generalized-gradient approximation using the projector augmented wave (PAW) method with the ABINIT code. We computed the free energies of hcp, bcc and fcc phases for iron and nickel including the thermal excitation of electrons and quasiharmonic phonons computed using the supercell method. We find that at high temperatures and pressures random stacking of fcc and hcp (rhcp) is most stable since the difference in Gibbs free energy between fcc and hcp Fe is smaller than the thermal energy. We computed the free energy of stacking faults, the rhcp phase and the percentage of fcc and partial ordered sequences along the melting curve. For the first time, we used first-principles calculation to be able to produce the pure-Fe phase diagram including the magnetic contributions at low pressures. In the Fe-Ni system, we find that FeNi3 is an intermediate phase below 700K at 0 GPa, consistent with experiment, and below 50 GPa at 0K. Finally, we obtained the T-P-x phase diagram for FeNi from 0-400 GPa and 0-6000K. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q13.00002: Plasticity-Mediated Structural Transformation of bcc Ta: Bridging Laser Heated Diamond-Anvil Cell and Shock Melting Christine Wu, Per Soderlind, James Glosli, John Klepeis Determination of the melting curve of a metal under high pressures is essential for establishing its phase diagram, and has wide scientific implications, including our understanding of the Earth's interior. Currently, melting temperatures at high pressure are primarily measured by \textit{in situ} laser-heated diamond-anvil cell (DAC) or shock wave experiments. Often, but not always, these two methods yield significantly different results for metals with non close-packed structures, such as bcc metals. For instance, anomalously flat melting slopes were reported for numerous bcc metals by DAC. The flatness of the melting slope is in sharp contrast to the classical Lindemann behavior which shock-melting temperatures follow closely. In this presentation, we will report our finding of a plasticity-mediated structural transition of bcc Ta to a partially disordered glassy structure obtained from molecular dynamics (MD) simulations. This transition is fully consistent with reported DAC low melting, thus provide a highly probable resolution to the long-standing controversy in melting of metals under high pressures. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q13.00003: High Pressure-High Temperature Phase Diagram of Beryllium M.J. Lipp, B.J. Baer, H. Cynn, Z. Jenei, J.-H. Klepeis, W.J. Evans, H.-P. Liermann, Y. Meng, S.V. Sinogeikin, W. Yang, A. Lazicki, Y. Ohishi A detailed understanding of the phase diagram of beryllium and its alloys impacts fundamental science and technological applications. Despite a simple atomic structure, theoretical modeling of the phase diagram of beryllium has been extremely challenging and remains an area of active investigation [Kadas, ,PRB 07]. Extension of the experimental understanding of beryllium will serve to inform and advance theoretical efforts and technological applications. To address these needs, we have extended our previous work [Evans, PRB 05], and performed x-ray diffraction and melt studies beryllium and beryllium alloys at high pressure. We will describe our measurements of the crystal structure, lattice constants, and melt curve of high-pressure beryllium and beryllium alloys. We will discuss insights into this simple yet challenging system. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q13.00004: High Pressure-Temperature Studies of Vanadium Z. Jenei, B.J. Baer, H. Cynn, J.-H. Klepeis, M.J. Lipp, W.J. Evans, H.-P. Liermann, Y. Meng, S.V. Sinogeikin, W. Yang Vanadium, a seemingly simple metal, has captured the interest of high-pressure scientists following the discovery (Ding et al. PRL 2007) of a subtle pressure-induced phase transition from bcc to a rhombohedral phase. Recent first-principles electronic-structure studies (Lee et al. PRB 2007) are consistent with these experiments and extend beyond the range of the measurements, predicting a reentrant phase transition back to bcc at high pressure. Further experiments in the regime of these predictions can validate and advance the understanding of simple metals at high-pressures. We have made x-ray diffraction measurements of the crystal structure and lattice parameters of vanadium at high-pressure and temperature. Detailed comparisons will challenge/validate models and guide development of predictive codes. We will discuss our measurements including high temperature behavior, the EOS, and transitions of vanadium at high pressure. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q13.00005: Coupling of Atomistic and Meso-scale Phase-field Modeling of Rapid Solidification J. Belak, P.E.A. Turchi, M.R. Dorr, D.F. Richards, J.-L. Fattebert, M.E. Wickett, F.H. Streitz Recently, phase field models have been introduced to model the crystallography during polycrystal microstructure evolution [1,2]. Here, we assess these models with molecular dynamics and phase field simulations that overlap in time and space. Large parallel computers have enabled MD simulations of sufficient scale to observe the formation of realistic microstructure during pressure driven solidification [3]. We compare the two methods by calculating the phase field order parameter (quaternion) from the atomic coordinates and drive the evolution with the MD. Results will be presented for the solidification of tantalum. [1] R. Kobayashi and J.A. Warren, Physica A, \textbf{356}, 127-132 (2005). [2] T. Pusztai, G. Bortel and L. Granasy, Europhys. Lett, 71, 131-137 (2005). [3] F. H. Streitz, J. N. Glosli, and M. V. Patel, Phys. Rev. Lett. 96, 225701 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q13.00006: New high-pressure phases of calcium and their finite-temperature phase boundaries Amanuel Teweldeberhan, Stanimir Bonev The phase diagram of Ca has been studied using first-principles density functional theory. The simple cubic structure hitherto believed to exist between 32 and 109 GPa is found to be mechanically and thermodynamically unstable. Instead we propose two new solid phases with orthorhombic {\it Cmcm} and {\it Pnma} structures and determine their finite-temperature phase boundaries. We also predict liquid transitions in molten Ca under compression, which together with the new solid phases provide a consistent description of the Ca phase diagram. The implications of our findings and extensions of the work to other alkali and alkaline-earth metals will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q13.00007: Physical and Chemical Transformations of Sodium Cyanide at High pressures Jing-Yin Chen, Choong-Shik Yoo Pressure-induced physical and chemical transformations of Sodium Cyanide (NaCN) have been studied up to 50 GPa in diamond-anvil cells, using micro-Raman spectroscopy and angle-resolved synchrotron x-ray diffraction. The present results suggest three phase transitions to occur in this pressure range: from NaCN-I (cubic) to NaCN-II (orthorhombic) at 2 GPa, to NaCN-III (monoclinic) at 8 GPa, and to NaCN-IV (tetragonal) at 15 GPa. At higher pressures, NaCN-IV undergoes irreversible chemical changes, which occurs over a large pressure range between 25 and 34 GPa. The new material exhibits a broad yet strong Raman band at around 1600 cm$^{-1}$, indicating the formation of C=N bonds in a similar configuration of carbon graphite. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q13.00008: First-order liquid-liquid phase transition in compressed nitrogen Brian Boates, Stanimir Bonev We present results of first-principles molecular dynamics simulations, which provide evidence for the existence of a first-order liquid-liquid phase transition in compressed nitrogen [1]. The transition is from a molecular to a polymeric liquid. It is characterized by a discontinuous loss of molecular stability followed, upon further compression, by gradual transformation until the local order of the liquid becomes similar to that of cg-N. We have computed the phase boundary of the liquid-liquid transition to be first-order between 2000 and 4000 K and determined that above 4000 K it becomes continuous. Comparison with measurements and suggestions for experimental confirmation of our predictions will be discussed as well. [1] B. Boates and S.A. Bonev, submitted. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q13.00009: High-temperature high-pressure properties of silica from Quantum Monte Carlo and Density Functional Perturbation Theory R.E. Cohen, K. Driver, Z. Wu, B. Militzer, P.L. Rios, M. Towler, R. Needs We have used diffusion quantum Monte Carlo (DMC) with the CASINO code with thermal free energies from phonons computed using density functional perturbation theory (DFPT) with the ABINIT code to obtain phase transition curves and thermal equations of state of silica phases under pressure. We obtain excellent agreement with experiments for the metastable phase transition from quartz to stishovite. The local density approximation (LDA) incorrectly gives stishovite as the ground state. The generalized gradient approximation (GGA) correctly gives quartz as the ground state, but does worse than LDA for the equations of state. DMC, variational quantum Monte Carlo (VMC), and DFT all give good results for the ferroelastic transition of stishovite to the CaCl$_2$ structure, and LDA or the WC exchange correlation potentials give good results within a given silica phase. The $\Delta V$ and $\Delta H$ from the CaCl$_2$ structure to $\alpha$-PbO$_2$ is small, giving uncertainly in the theoretical transition pressure. It is interesting that DFT has trouble with silica transitions, although the electronic structures of silica are insulating, simple closed-shell with ionic/covalent bonding. It seems like the errors in DFT are from not precisely giving the ion sizes. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q13.00010: Quantum Monte Carlo Equations of State of $\alpha$- and $\beta$-Magnesium Silicate Kevin P. Driver, John W. Wilkins The 410 km seismic discontinuity in Earth's mantle is ascribed to the $\alpha$ to $\beta$-(Mg,Fe)$_2$SiO$_4$ phase transformation. Considering Mg-endmembers, density functional theory (DFT) predictions within LDA and GGA disagree on the phase boundary by 50\% [1]. Quantum Monte Carlo (QMC) offers a route to avoid the approximation of the exchange-correlation potential in DFT and provide a benchmark for the phase boundary, elastic moduli, and thermodynamic properties. Zero point and thermal contributions are included by using DFT linear response within the quasiharmonic approximation. Preliminary results indicate the QMC phase relations and bulk moduli are in reasonable agreement with experiment. \\[4pt] [1] Y. Yu, Z. Wu., R. M. Wentzcovitch, Earth Planet. Sci. Lett. 273, 115 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q13.00011: High Pressure Studies on Group IV Transition Metals Based Metallic Glasses Andrew Stemshorn, Yogesh Vohra The compression behavior of Group IV transition metals based metallic glasses Ti37Zr29Cu15.5Ni14.5Be4 and Zr57Cu15.4Ni12.6Al10Nb5 are investigated at room temperature up to 74 GPa in a diamond anvil cell using in-situ energy and angular dispersive x-ray diffraction with a synchrotron radiation source. The x-ray diffraction studies did not reveal any pressure induced crystallization phenomenon in metallic glasses to a volume compression of 35 percent. In Zr-based metallic glass, a nanostructured tetragonal Zr2Ni phase was observed and also found to be stable to the highest pressure. The measured equation of state (Pressure-Volume curve) of Group IV transition metals based metallic glasses is compared to the known high phases of transition metals. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q13.00012: High pressure-High temperature phases of Carbon Dioxide Amartya Sengupta, Choong-Shik Yoo The phase diagram of CO$_{2}$ has not been understood adequately above 40 GPa and high temperatures, particularly regarding the stabilities and boundaries of various extended phases that include a-carbonia, Phase V, Phase VI, and to an extent Phase III. We have studied the phase diagram of CO$_{2}$ above 40 GPa and at high temperatures, using both ohmically and laser-heated diamond anvil cells. We found the co-existence of several extended phases over a large pressure region, which we attribute to the metastability of the extended phases and the extraordinarily large pressure gradients at these pressures. We determined the relative stability fields of the co-existing phases, which may offer the physico-chemical mechanism for the existence of carbonate minerals in deep Earth's mantle. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q13.00013: First-principles study of BC$_2$N Eunja Kim, Tao Pang, Wataru Utsumi, Vladimir Solozhenko, Yusheng Zhao First-principles calculations are performed and analyzed to identify different cubic phases of BC$_2$N synthesized experimentally. With a proper choice of the supercell, cutoff energy, and sampling $k$ points, the cubic phases are found to be stable theoretically. The bulk modulus from elastic stiffness constants for each of the phases is in excellent agreement with available experimental data. All the phases are defect free and do not possess any B--B or N--N bond. Two high-density phases with nearly degenerate energies are interpreted to represent two experimental systems of different x-ray patterns. The high-density phases are characterized by the existence of C--C bonds whereas the low-density phase is characterized by the absence of C--C bonds. From the calculated equation of state and the available experimental data, we show for the first time that the unique feature of each of the cubic BC$_2$N phases is a direct result of the corresponding local electronic structure and chemical bonding in the system. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q13.00014: Large volume change across OI --$>$ OII phase transition in transition-metal dioxides TiO$_{2}$, ZrO$_{2}$, and HfO$_{2}$ as determined by experiment and theory Yahya Al-Khatatbeh, Kanani K.M. Lee, Boris Kiefer The nature of bonding in transition-metal dioxides TiO$_{2}$, ZrO$_{2}$, and HfO$_{2}$ is of interest as they are potential superhard materials with many industrial applications. Using high-resolution synchrotron x-ray powder diffraction for TiO$_{2}$ and ZrO$_{2}$, and complementary \textit{ab-initio} computations of these dioxides, we have determined the equation of state of the orthorhombic I (OI) and orthorhombic II (OII) phases. Our measurements are in agreement with the computationally predicted phase sequence of these oxides. The measured volume change across OI --$>$ OII transition is 8.3{\%} for TiO$_{2}$ and 10{\%} for ZrO$_{2}$ in good agreement with our density-functional theory (DFT) calculations that predict a large volume change for all of these dioxides across the OI --$>$ OII phase transition. For TiO$_{2}$, this volume collapse is significantly higher than previously measured (2.6{\%}), but consistent with the volume decreases observed in both ZrO$_{2}$ and HfO$_{2}$ across this transition. Furthermore, the OII phase was observed to be the most stable phase of TiO$_{2}$ and ZrO$_{2}$ at high pressure (56 GPa) after heating to high temperatures (above $\sim $1800 K) and no post-OII phase was observed under these conditions. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q13.00015: Crystal Structure and Phase Transition of XeF$_{2}$ at High pressures Minseob Kim, Choong-Shik Yoo We have investigated the crystal structure and phase transition of solid XeF$_{2}$ up to 51 GPa in diamond anvil cells by using Raman and synchrotron x-ray diffraction. The x-ray data indicates the tetragonal-to-orthorhombic phase transition at 7 GPa, which accompanies a small distortion ($<$ 1{\%}) in the ab-plane of the tetragonal structure. The Rietveld refinement further indicates a rapid change of intermolecular F...F contact distance with increasing pressures and, thereby, a rotation of linear symmetric XeF$_{2}$ molecules along the c-axis and the observed distortion in the ab-plane. This symmetry lowering tetragonal-to-orthorhombic transition also induces the Davydov splitting of symmetric stretching $\nu _{1}$ and bending $\nu _{2}$ modes in the Raman spectrum. [Preview Abstract] |
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