Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G26: Focus Session: Materials in Extremes: Phase Transitions |
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Sponsoring Units: GSCCM DCOMP DMP Chair: Renata Wentzcovich, University of Minnesota Room: 502 |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G26.00001: Computation of temperature induced phase transitions at high pressure Invited Speaker: Anatoly Belonoshko Phase transitions at high pressure and temperature is perhaps one of the most controversial topics in high pressure science. There is a number of theoretical methods developed recently to address this problem. Unfortunately, none of them is fully satisfactory, considering stringent requirements to accuracy of the computed free energies of the involved phases. This, in part, explains the variety of the suggested methods. The experimental data is rather controversial as well, probably because the experiments at extreme conditions are difficult. I will provide overview of the theoretical methods that are applied for the computation and simulation of T-induced phase transitions. Both liquid-solid and solid-solid transitions will be covered. Possible sources of the disagreement between theoretical methods as well as between theory and experiment will be illustrated by examples. Insight from simulations will be used to suggest alternative explanations of experimental data. The optimal, at present, method to compute the transitions will be suggested. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G26.00002: Ab-inition melting curve of titanium Vincent Stutzmann, Johann Bouchet, Francois Bottin Thermodynamical properties of titanium are of great interest for aerospace and aviation industries and many studies are done in order to understand its behaviour under pressure (P) and temperature (T) : phase transitions at low T, melting curve at high T and P. In this work we compute the first \textit{ab-initio} melting curve of titanium. This one is obtained with the \textsc{Abinit} package using DFT, in the GGA approximation, and in the framework of the projector augmented wave method (PAW). At first, we perform ground state calculations and study the five allotropic phases of titanium. Two PAW atomic data are generated with two different cutoff radius. The larger one gives results near previews \textit{ab-initio} calculations, whereas the smaller one gives results near all electron calculation. Using the second PAW atomic data and performing \textit{ab-initio} molecular dynamic simulations, we then compute the melting curve of titanium with three different methods. Results show relevance of our calculations, but also discrepencies with experimental data. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G26.00003: Is Orthorhombic Iron Tetraboride Superhard? Bo Xu, Qianqian Wang, Julong He, Yongjun He Millimeter-sized FeB$_{4}$ bulks were synthesized under high pressure of 15 GPa and 1600 $^{\circ}$C. Multiple analysis methodologies including XRD, SEM, TEM, and Raman spectroscopy verified this new phase with an oP10 crystal structure. A relatively low hardness value of 15.4 GPa, which is consistent with both macroscopic and microscopic hardness models, excludes FeB$_{4}$ as a superhard material. Resistance and magnetization measurements do not indicate superconductivity in FeB$_{4}$for temperature as low as 2.5 K. Previously reported superconductivity in FeB$_{4}$ needs a further investigation, especially for structural imperfections, unidentified phases, and/or contaminations. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G26.00004: Solidification and fcc- to metastable hcp- phase transition in krypton under modulating dynamic pressures Jing-Yin Chen, Choong-Shik Yoo, Minseob Kim, Hanns-Peter Liermann, Hyunchae Cynn, Zsolt Jenei, William Evans We describe high-pressure kinetic studies of the solidification, melting and fcc-hcp transitions of Krypton under dynamic loading conditions, using a \textit{dynamic}-diamond anvil cell ($d$-DAC) coupled with time-resolved x-ray diffraction. The time-resolved diffraction patterns and dynamic pressure responses show compression-rate dependencies associated with both the decay and growth time constants of the liquid-solid and solid-liquid transitions. According to the Avrami equation, both the solidified and melting processes are spontaneous nucleation and a rod-like (1-D) growth. Furthermore, under dynamic loading conditions, Kr-hcp forms from fcc close to the melting line. The nucleation time of fcc and hcp are very fast, with little dependence of compression rates or shorter than the time resolutions. The threshold pressure for the formation of Kr-hcp is $\sim$ 0.8 GPa at the dynamic loadings of 0.004-13 GPa/s. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G26.00005: Compositional changes upon compression of sodium azide predicted using density functional theory Brad Steele, Aaron Landerville, Ivan Oleynik The pressure induced phase transitions in sodium azide, which include a potential polymeric nitrogen phase transition, are investigated using evolutionary crystal structure prediction methods coupled with density functional theory calculations. Two new phases are predicted to be stable above 53 GPa that have an inequivalent ratio of sodium to nitrogen atoms as compared to sodium azide. The Raman spectrum is calculated from 0-100 GPa using these newly predicted structures, as well as the newly discovered I4/mcm phase of sodium azide. The predicted Raman spectrum is shown to give good agreement to experimental data above 30 GPa and below 15 GPa. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G26.00006: Phase transition in liquid hydrogen at high pressure and temperature Katsuya Shimizu, Kenji Ohta, Kota Ichimaru, Takahiro Matsuoka, Yasuo Ohishi, Naohisa Hirao The heating rate measurements indicated the existence of a transition in the liquid phase of hydrogen. A gold thin foil was loaded into the sample chamber of DAC with liquid hydrogen and compressed up to 100 GPa. The gold foil was heated by IR laser and the temperature was measured by the radiation. The anomaly was found on the temperature curve as a function of the laser power which indicates a transition at the temperature. The absorptance change indicates the heat flow rate change from the gold foil to the diamond surface through the hydrogen. The condition of the pressure and temperature of the transition is good agreement with the previous theoretical and experimental reports. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G26.00007: New Phase Transition in High Pressure Molecular Oxygen Yanier Crespo, Sandro Scandolo, Erio Tosatti Oxygen, one of the most common and important elements in nature, has an exceedingly well explored phase diagram under pressure, up and beyond 100 GPa. We propose a subtle, yet surprisingly undetected pressure-induced phase transition of molecular O$_2$ within the $\epsilon$ phase, near 20 GPa. While thus far in the whole $\epsilon$ phase region from about 8 to 96 GPa each individual O$_2$ molecule was generally believed to be in an S=0 state, we provide theoretical results, strongly connecting with existing vibrational and structural evidence, showing that this holds only above 20 GPa, whereas there is a collective switch from S=0 to an S=1 molecular state at and below 20 GPa. The former $\epsilon$ phase thus breaks up into two: a spinless $\epsilon_0$ (20-96 GPa), and $\epsilon_1$ (8-20 GPa) where the molecules possess spin with at least short range antiferromagnetic correlations. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G26.00008: Novel Si networks in Ca/Si phase diagram under pressure Guoying Gao, Neil Ashcroft, Roald Hoffmann In the Ca/Si phase diagram, many compositions are known. In these calcium silicides, silicon atoms form many different organizations, for example, at low pressure silicons are isolated silicon atoms in Ca$_{\mathrm{2}}$Si, Si chains in CaSi and corrugated hexagonal Si layers and a three-dimensional network of \textit{sp}$^{\mathrm{2}}$ bonds in CaSi$_{\mathrm{2}}$. The crystal structures for these silicides under pressure have not been studied completely, and we are very interested in the new chemical and physical behavior of Si in these silicides under pressure. Therefore, we take a theoretical study of Ca$_{\mathrm{2}}$Si, CaSi and CaSi$_{\mathrm{2}}$ under pressure. We predicted many interesting Si networks in the calcium silicides under pressure. Si atoms form Si chains in Ca$_{\mathrm{2}}$Si, flat quadrangular and hexagonal Si layers in CaSi, and 6-coordinated Si tetrahedrons and 4, 8-coordinated Si octahedrons in CaSi$_{\mathrm{2}}$ at high pressure. All of these predicted structures are dynamically stable. Moreover, these calcium silicides are all metals. Some of them are good candidates to be superconductors. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G26.00009: High pressure Raman spectroscopy of phase change materials Wen-Pin Hsieh, Peter Zalden, Matthias Wuttig, Aaron Lindenberg, Wendy Mao We used high-pressure Raman spectroscopy to study the evolution of vibrational frequencies of the phase change materials (PCMs) Ge$_{2}$Sb$_{2}$Te$_{5}$, GeSb$_{2}$Te$_{4}$, and SnSb$_{2}$Te$_{4}$. We found that the critical pressure for triggering amorphization in the PCMs decreases with increasing vacancy concentration, demonstrating that the presence of vacancies, rather than differences in the atomic covalent radii, is crucial for pressure-induced amorphization in PCMs. Compared to the \textit{as-deposited} amorphous phase, the \textit{pressure-induced} amorphous phase has a similar vibrational spectrum, but requires much lower laser power to transform into the crystalline phase, suggesting different kinetics of crystallization, which may have implications for applications of PCMs in non-volatile data storage. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G26.00010: Pressure-Induced Amorphization in Single-Crystal Ta$_{2}$O$_{5}$ Nanowires: A Kinetic Mechanism and Improved Electrical Conductivity Xujie Lu, Qingyang Hu, Wenge Yang, Ligang Bai, Howard Sheng, Lin Wang, Fuqiang Huang, Jianguo Wen, Dean Miller, Yusheng Zhao Pressure-induced amorphization (PIA) in single-crystal Ta$_{2}$O$_{5}$ nanowires is observed at 19 GPa and the obtained amorphous Ta$_{2}$O$_{5}$ nanowires show significant improvement in electrical conductivity. The phase transition process is unveiled by monitoring structural evolution with in-situ synchrotron XRD, PDF, Raman spectroscopy and TEM. The first principles calculations reveal the phonon modes softening during compression at particular bonds, and the analysis on the electron localization function also shows bond strength weakening at the same positions. Based on the experimental and theoretical results, a kinetic PIA mechanism is proposed and demonstrated systematically that amorphization is initiated by the disruption of connectivity between polyhedra at the particular weak-bonding positions along the a-axis in the unit cell. The one-dimensional morphology is well preserved for the pressure-induced amorphous Ta$_{2}$O$_{5}$ and the electrical conductivity is improved by an order of magnitude compared to traditional amorphous forms. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G26.00011: Nanowire Ice of Phase VI and Distorted VII in Mesoporous Silica Nanotorus Superlattice Jinlong Zhu, Jianzhong Zhang, Yusheng Zhao The motivation of nano H$_{2}$O realization and characterization is the highly polarized nature of H$_{2}$O molecules and the spatial hydrogen bonded networks both in liquid and solid form. The hydrogen bonding character of water molecules results in a remarkably rich phase diagram in the pressure-temperature space. Water/Ice confined in nanochannels showed novel structures and properties as results of hydrophobic and hydrophilic interactions and hydrogen bonding interaction between water molecule and the surface of nanochannel. Studies on nano H$_{2}$O can provide potential pathway to understand the complicated structure evolutions of ice in the $P$-$T$ space, because the interplay between nano-confinement and strong intermolecular hydrogen interactions can lead to even richer ice structures which were not found in the none-confined bulk form. The high pressure experiment indicated that the pressure of nanowire ice VI and VII shifted up to 1.7 GPa and 2.5 GPa, and about $\sim$ 0.65 GPa and 0.4 GPa higher than that of normal ice. The nano size effect and the strength of mesoporous silica nanotorus are responsible for the pressure shifts of ice phase regions. More pronounced, the cubic ice VII changed into a tetragonal distorted ``psuedocubic'' structure of the nanowire ice when confined in the mesoporous tubes. The degree of tetragonality increased with increasing pressure, which is resulted from the uniaxial pressure nanowire ice felt, and the anisotropic hydrogen bonding interactions including the H$_{2}$O-H$_{2}$O hydrogen bonds in the bulk of the ice and the H$_{2}$O-silica --OH hydrogen bonds between the interface of nanowire ice and mesoporous silica. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G26.00012: Giant pressure-induced volume collapse in the pyrite mineral MnS2 Simon Kimber, Ashkan Salamat, Shaun Evans, Harald Jeschke, Kaliappan Muthukumar, Milan Tomic, Francesc Salvat-Pujol, Roser Valenti, Maria Kaisheva, Ivo Zizak, Tapan Chatterji Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high temperature superconductivity. In transition metal materials, collapses are usually driven by so-called ``spin state'' transitions- the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S= 5/2 mineral Hauerite (MnS2) undergoes an unprecedented 22\% volume collapse driven by a conceptually new magnetic mechanism. Using synchrotron x-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic ground state using in-situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S= 1/2 moments are quenched by dimerisation. Our results show how the emergence of metal-metal bonding can stabilise giant spin-lattice coupling in Earth's minerals. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G26.00013: Nonhysteretic superelasticity of shape memory alloys at nanoscale Zhen Zhang, Xiangdong Ding, Xiaobing Ren, Kazuhiro Otsuka Superelasticity with recoverable strains about 8{\%} is known to appear in shape memory alloys (SMAs) possessing a spontaneous martensitic transformation (MT). However, it is unknown whether such a property can still exist below the critical size where MT disappears. We perform molecular dynamics simulations to show that SMA nanoparticles below the critical size not only demonstrate superelasticity but also exhibit features such as absence of hysteresis, continuous nonlinear elastic distortion, and high blocking force. Atomic level investigations show that this nonhysteretic superelasticity results from a continuous transformation from the parent phase to martensite under external stress. This aspect of SMAs is attributed to a surface effect; i.e., the surface locally retards the formation of martensite and then induces a critical-end-point-like behavior when the system is below the critical size. Our work potentially broadens the application of SMAs to the nanoscale. It also suggests a method to achieve nonhysteretic superelasticity in conventional bulk SMAs. [Preview Abstract] |
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