Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T15: Metals, Compounds, and Metal-like Behavior |
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Sponsoring Units: DCMP Chair: Joerg Neugebauer, Max-Planck Institut fur Eisenforschung Room: B114 |
Wednesday, March 17, 2010 2:30PM - 2:42PM |
T15.00001: An initial analysis of short- and medium-range correlations potential non-Pt catalysts in CoNx Joe Peterson A potential show stopper for the development of fuel cells for the commercial automotive industry is the design of low-cost catalysts. The best catalysts are based on platinum, which is a rare and expensive noble metal. Our group has been involved in the characterization of potential materials for non-Pt catalysts. In this presentation, I will present some preliminary neutron scattering data from a nanocrystalline powder sample of CoNx. It is apparent that the diffraction data cannot be analyzed with standard Riedveld refinement, and we have to invoke pair distribution function (PDF) analysis. The PDF provides insight into short-range correlations, as it measures the probabilities of short- and mid-range interatomic distances in a material. The analysis reveals a strong incoherent scattering response, which is indicative of the presence of hydrogen in the sample. After correcting for the incoherent scattering, one obtains the normalized scattering function S(Q), whose Fourier transform yields the PDF. [Preview Abstract] |
Wednesday, March 17, 2010 2:42PM - 2:54PM |
T15.00002: ABSTRACT WITHDRAWN |
Wednesday, March 17, 2010 2:54PM - 3:06PM |
T15.00003: Kondo Fluctuation Signature on the Magnetocaloric Effect in the YbInCu$_{4}$ Under Pressure A.L. Lima Sharma, A.M. Gomes, Catalina Salazar Mejia, F.R. Drymiotis, A.M.G. Carvalho We have found a remarkable signature of the Kondo effect in the magnetocaloric effect MCE of YbInCu$_4$. The compound YblnCu4 presents a well-observed valence transition near 42K from free-moment to a low-temperature metallic mixed-valence phase. The magnetocaloric effect (MCE), $\Delta $S, presents several features that can be understood as signatures of Kondo effect, valence transition and crystal electric field effects. Under pressure, the MCE not only increases in absolute value of the isothermal entropy change, but also shifts the low temperatures towards the Kondo temperature. We measured the heat flux using a differential scanning calorimeter in order to estimate the internal pressure of the local moment phase over the itinerant one, near the vicinity of the transition temperature. Moreover, we found that this compound, when submitted to a thermodynamic cycle, follows a nearly reversible process. [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:18PM |
T15.00004: Groundstates in AgPd using cluster expansion methods Lance Nelson, Gus Hart Palladium alloys have applications in electronics, catalysis, hydrogen storage, and jewelry. The appearance of ordered phases can improve the materials properties for some applications. We present a study of the Ag-Pd system, which is of current interest in electronics and jewelry. We use a model Hamiltonian to quickly search large sets of derivative superstructures for ground states. The model Hamiltonian allows us to perform Monte Carlo simulations to determine transition temperatures. Previous predictions show two unusual ground states: L1$_1$, which has only been observed in Cu-Pt, and a new structure (never observed in any other system) with a 1:3 stoichiometry (dubbed L1$_3$). We find that L1$_3$ is actually unstable. We also predict the ordering temperature for the L1$_1$ phase. [Preview Abstract] |
Wednesday, March 17, 2010 3:18PM - 3:30PM |
T15.00005: Structural Transitions in IrV and RhV Michael Mehl, Stefano Curtarolo, Gus Hart The orthorhombic $\alpha$IrV phase is only observed in the intermetallics IrV and RhV, where it appears in an apparently second-order phase transition from the tetragonal $L1_0$ phase upon cooling below 500$^\circ$C.\footnote{B. Chen and H. F. Franzen, J Less-Common Metals {\bf 159} 343-7 (1990).} It has been suggested that this transition is driven by an unstable phonon at the R point of the $L1_0$ Brillouin zone.\footnote{H. F. Franzen, Chem. Mater. {\bf 2}, 486-491 (1990).} First principles calculations show that all of the phonon modes at R are stable in both materials. We do find a region of unstable phonons near and along the line ($\frac14 y \frac12$). None of these modes relaxes directly into the $\alpha$IrV structure. The lowest, at $y = \frac14$, relaxes the system into the Ga$_3$Pt$_5$ structure, which is itself vibrationally unstable at the $\Gamma$ point, relaxing into a structure with symmetry Amm2 which has not been seen in any intermetallic. We follow this pathway of phonon instabilities to lower energy structures. We also discuss the possibility that the structural transition might be first-order, rather than second-order as indicated. [Preview Abstract] |
Wednesday, March 17, 2010 3:30PM - 3:42PM |
T15.00006: Short- and Medium-Range Order in Amorphous Zr$_{2}$Ni Metallic Alloy Li Huang, Cai-Zhuang Wang, Shao-Gang Hao, Matthew J. Kramer, Kai-Ming Ho Icosahedral clusters are commonly believed to be the key building blocks in many metallic glasses. Here we propose a structural model for Zr$_{2}$Ni metallic glass which has a small fraction of icosahedral clusters. By analyzing the correlation between the local structure and dynamics in the undercooled liquid and glass, we show that the formation of metallic glass in this system can be attributed to the slow dynamics of $<0,2,8,1>$ and $<0,2,8,2>$ Ni-centered Voronoi polyhedra. There is a high proportion of less mobile $<0,2,8,4>$, $<0,2,8,5>$, $<0,1,10,4>$, and $<0,1,10,3>$ Zr-centered clusters on the first shell of the two types of Ni-centered clusters. These special Ni- and Zr-centered clusters arrange together to form a string-like backbone network in the metallic glass. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T15.00007: Electronic mechanism of shape memory behavior in NiTi-based ternary alloys N. Hatcher, A. J. Freeman, Oleg Y. Kontsevoi While the electronic foundations of shape memory behavior in NiTi have been thoroughly examined and elucidated\footnote{Hatcher,\,Kontsevoi,\,and\,Freeman,\,Phys.\,Rev.\,B,$\bf79$,\,020202(R)\,(2009)}, we extend this understanding to ternary alloys. The effect of ternary additions on the martensitic behavior of NiTi is investigated by applying {\it ab initio} calculations with the highly precise FLAPW method to the Ni-Ti-X(X=Pt, Pd; 0-30$\%$) system. We determine ternary element site preferences, pair interaction energies, and the energy hierarchy among the phases, finding that Pd and Pt atoms replace Ni and decorate the lattice at second and third nearest neighbors from one another, respectively. By calculating detailed elastic properties, cleavage energies, and planar generalized stacking fault energetics, we explain brittle/ductile behavior in the system, identify slip systems, and find that the C$'$ elastic constant becomes unstable with increased alloying. Finally, we establish how ternary elements affect the physics governing martensitic behavior by tracing changes in Fermi surface nesting behavior to mechanical instabilities and identifying unstable atomic vibration modes through imaginary branches of calculated phonon dispersions. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T15.00008: Sensitivity of the Stacking Fault Energy in FeMn Alloys on the Local Environment: a First-Principles Study Alexey Dick, Tilmann Hickel, J\"{o}rg Neugebauer An in-depth understanding of the physical processes that may influence the stacking fault energy (SFE) is necessary for a knowledge-based optimization and engineering of high-Mn-steels. We have performed a first-principles study of the SFE in austenitic FeMn-alloys, which are prototype structures for realistic high-Mn-steels. The relevant atomic configurations have been identified by combination of the cluster-expansion methodology and the concept of special quasirandom structures based on the density functional theory calculations. Employing either the axial interaction model and/or explicit calculations of the generalized SFE surfaces we show that the value of the SFE sensitively depends on type of the chemical and magnetic ordering in the system. We further show that the SFE can be changed not only by varying the composition of the FeMn-alloy or its temperature, but also by admixing different chemical elements or by controlling local strain fields. [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T15.00009: New structures in Pd-rich ordered alloys Jacqueline Corbitt, Erin Gilmartin, Gus Hart An intriguing intermetallic structure with 8:1 stoichiometry was discovered in 1959 in the Pt-Ti system. Since then a handful of other Pt/Pd/Ni binary systems have been observed to exhibit this structure (Pt8Zr, Pd8Mo, Ni8Nb, etc). Precipitates of this ordered structure can significantly increase the hardness of an alloy. For jewelry applications involving Pt and Pd, international hallmarking standards require that the alloys be at least 95\% pure by weight. However, Pt- and Pd-rich alloys are often too soft for jewelry applications when purity is high and the minority atoms are disordered. Because the 8:1 structure maintains a high weight percentage of Pt/Pd, it can satisfy hallmarking while increasing performance. Recent calculations and experiments suggest that the 8:1 structure may form in about 20 previously unsuspected Pt/Pd binary systems. Using first-principles calculations and cluster expansion modeling, we have performed a ground state search to find the stable structures in Pd-Nb and Pd-Cu. In collaboration with Candace Lang's group at University of Capetown South Africa, we are working to experimentally validate the predicted ground states. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T15.00010: Phonon Anomalies and Metal Insulator Transition in Fe(1-x)Co(x)Si Olivier Delaire, Matthew Lucas, Matthew Stone, Douglas Abernathy, Karol Marty, Paul Kent, Brian Sales, David Mandrus The Fe(1-x)Co(x)-Si ordered compound (B20 structure) undergoes a metal-insulator transition upon doping with Co or heating. FeSi is a narrow band-gap semiconductor, whereas CoSi is a metal. Phonons were measured on both single crystals and powders as function of composition and temperature, using inelastic neutron scattering. A reciprocal-space time-of-flight tomography technique, as well as conventional triple-axis spectrometry, were used to map extensive regions of the FeSi dispersions. The phonon branches in FeSi exhibit an excess softening compared to those of CoSi, which appears in better agreement with a pure volume effect. Using first-principles electronic structure calculations and ab-initio molecular dynamics, the anomalies are explained in terms of a metallization induced by thermal disorder. This effect is also related to other cases where the electronic structure leads to anomalous temperature dependencies of the phonons. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T15.00011: {\it Ab initio} equation-of-state and elastic properties of Pu metal and Pu-Ga alloys Per Soderlind, Alex Landa We present results of {\it ab initio} calculations of equation-of-state and elastic properties for Pu metal and Pu-Ga alloys. For this we have employed density-functional theory (DFT) in conjunction with spin-orbit coupling and orbital polarization for the metal and coherent-potential approximation (CPA) for the alloys. All Pu systems benefit from spin polarization which is consistent with previous DFT studies of plutonium. We show that orbital correlations become more important proceeding from $\alpha \rightarrow \beta \rightarrow \gamma$ plutonium, thus suggesting increasing $f$-electron correlation (localization). For $\delta$-Pu- Ga alloys we find that the system softens with larger Ga content, i.e., bulk modulus, elastic constants, and chemical bonding weakens with increasing Ga concentration. This inverse relationship is nearly linear and supported by measurements on polycrystal $\delta$-Pu-Ga alloys. For Pu metal, our single-crystal results also relates reasonably with ultrasound data on polycrystal samples where available. The comparison is indirect but made possible by approximating the polycrystal with an isotropic (uniform strain) single crystal. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T15.00012: Electronic, structural, and thermodynamic properties of actinide dioxides Li Ma, Raymond Atta-Fynn, Asok K. Ray As a continuation of our studies of pure actinide metals using hybrid density functional theory,\footnote{R. Atta-Fynn and A. K. Ray, Europhysics Letters, \textbf{85}, 27008-p1- p6 (2009); Chemical Physics Letters, \textbf{482}, 223-227 (2009).} we present here a systematic study of the electronic and geometric structure properties of the actinide dioxides, UO$_{2}$, PuO$_{2}$ and AmO$_{2}$, using both density functional and hybrid density functional theories. For the hybrid density functionals, the fractions of exact Hartree-Fock exchange used were 25{\%} and 40{\%}. Each compound has been studied at the nonmagnetic, ferromagnetic and antiferromagnetic configurations, with and without spin-orbit coupling (SOC). The influence of SOC on the properties of the actinide dioxides will be discussed. Thermodynamic properties such as phonon dispersion curves, heat capacity, entropy, internal energy and free energy have been calculated by a coupling of first-principles calculations and lattice dynamics. [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T15.00013: Elastic Moduli of Beta Plutonium Yoko Suzuki, Victor R. Fanelli, Albert Migliori, Jonathan B. Betts, Jeremy M. Mitchell, Michael Ramos, Franz Freibert, Charles H. Mielke The elastic moduli of pure polycrystalline beta Pu were measured using resonant ultrasound spectroscopy between 415 K (above the transition from the alpha phase) and 491 K (below the transition to the gamma phase). Results will be compared with the elastic moduli of the alpha and gamma phases, measured on the same specimen, as well as the Gallium-stabilized delta phase. The bulk modulus lies between those of the alpha and gamma phases as expected, but it has unusually small temperature dependence unlike that previously reported for the beta phase. The shear modulus is surprisingly nearly continuous between the beta and gamma phases. The Poisson's ratio shows typical metallic behavior. Overall, the elastic moduli we measured have higher values than any previous measurements, an indication of the higher quality of our sample. [Preview Abstract] |
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