### Session Q40: Metals: Nanoparticles, Compounds and Thermodynamics

 Wednesday, March 12, 2008 11:15AM - 11:27AM Q40.00001: Sintering Behavior of Metallic Nanoparticles Martin Fendrich , Ralf Meyer , Ruslan Zinetullin , Dietrich E. Wolf , Peter Entel Novel techological applications call increasingly for the controlled production of nanoparticles with well-defined physical properties. An important subject in this field is the sintering of agglomerated particles. We employ two different computer-simulation techniques to simulate the sintering of metallic nanoparticles. Molecular-dynamics simulations are used to study the early stage ($t<\,\approx$100\,ns) of the sintering of two Ni nanoparticles with a size of about 4\,nm. These simulations make it possible to observe important metallurgical details, like the role of (partial) dislocations in the process of the alignment of the crystal lattices. The data of the molecular-dynamics simulations are then used to calibrate the time-scale of kinetic Monte-Carlo simulations which can follow the sintering process over a much longer time-scale. A special technique is used in these simulations which makes it possible to take the misaligned crystal lattices into account. Wednesday, March 12, 2008 11:27AM - 11:39AM Q40.00002: Molecular Dynamics Study of Phase Transitions of Transition Metal Alloy Nanoparticles Lijun Meng , Kaiwang Zhang , Lizhong Sui , Chao Tang , Huaping Xiao , G. Malcolm Stocks , Jianxin Zhong We have investigated structures of FeCo, FeNi and CoNi alloy nanoparticles using molecular dynamics simulations based on generalized embedded-atom potentials. The pair correlation function (PCF), the Honeycutt and Andersen index (HA index), and bond-order analysis techniques were employed for characterization of the atomic structures of the nanoparticles. Structural transitions in alloy nanoparticles in the size range of 300 to 800 atoms were identified through continuous variation of the compositions. We found that a FeCo nanoparticle exhibits a phase transition from a BCC structure to an icosahedral structure as the cobalt composition is larger than 60{\%}. FeNi nanoparticles possess a BCC or an icosahedral phase if nickel is smaller than 30{\%} or higher than 70{\%}, respectively. In the range of 30{\%}-70{\%} nickel, a FeNi nanoparticle shows features of both BCC and FCC phases. However, a CoNi nanoparticle always has an icosahedral structure in the whole composition range of Ni. Wednesday, March 12, 2008 11:39AM - 11:51AM Q40.00003: Computer simulation of nanoparticle aggregate fracture Brian Henz , Takumi Hawa , Michael Zachariah Nanoparticle aggregates have been found to possess unique mechanical properties. Aggregates of metal nanoparticles can be strained up to 100{\%} before failure, and even typically brittle materials are observed to have a ductile failure mode. In this effort two materials; namely silver and silicon, were chosen to represent ductile and brittle materials, respectively. Aggregates with 2 to 10 particles were simulated using the molecular dynamics (MD) algorithm to determine the stress-strain behavior of the aggregate. By comparing MD results with the AFM experiments of two sintered nanoparticles we have reaffirmed the observation that even brittle materials may behave as ductile materials at the nanoscale. Wednesday, March 12, 2008 11:51AM - 12:03PM Q40.00004: Preparation and Characterization of Chemically Prepared Fe-Ag Nanocomposites Thomas Ekiert , Gerald Poirier , Karl Unruh Fe-Ag nanocomposites have been prepared by a transmetallation reaction in which metallic Fe nanoparticles are used to reduce an aqueous solution of Ag$^{+}$ ions. As the reaction proceeds, the Fe nanoparticles fragment into smaller particles and are reduced in size and metallic Ag nanoparticles are formed. The evolution in the structure and composition of the reaction products has been studied as a function of the time and the solution temperature by scanning and transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction measurements. These measurements indicate that nearly all of the available Ag$^{+}$ ions are reduced to metallic Ag within 5 minutes at solution temperatures between 5 \r{ }C and 35 \r{ }C. However, at higher solution temperatures the fragments of the Fe nanoparticles very quickly oxidize if the reaction is not carried out under anaerobic conditions. The magnetic properties of the Fe-Ag nanocomposites have also been studied by vibrating sample magnetometry. These measurements show a corresponding decrease in the saturation moment of those Fe-Ag nanocomposites not prepared under anaerobic conditions, with the majority of the change occurring within the first five minutes of exposure to the Ag$^{+}$ solution. Wednesday, March 12, 2008 12:03PM - 12:15PM Q40.00005: Creation of low-energy twin lamellae for thermal stability in nanostructured materials Christopher Saldana , Tejas Murthy , Ravi Shankar , Srinivasan Chandrasekar , Eric Stach Intrinsic thermal instability of nanostructured metals have limited the applicability of these high-strength material systems. A novel stabilization route was discovered in these fine-grained systems when a high-density twin nano-lamella was introduced amongst nano-grain boundaries through SPD at cryogenic temperatures. The stabilization in such a composite microstructure was traced to the peculiar kinematic behavior of the twin-grain boundary triple junction. Copper was chosen as model material and deformed under cryogenic conditions using machining with varying deformation rates. The microstructure was investigated through HREM as a function of time and temperature. At small deformation rates, the SPD at cryogenic temperatures resulted in the creation of a nanostructured material with an unstable microstructure that coarsened even at room temperatures. At higher deformation rates under the same conditions, distribution of twin lamellae resulted in a thermally stable nanostructured material. Wednesday, March 12, 2008 12:15PM - 12:27PM Q40.00006: Electron-Phonon Interaction and High-Temperature Thermodynamics in Vanadium Alloys and Compounds Olivier Delaire , Max Kresch , Matthew Lucas , Jorge Munoz , Jiao Lin , Brent Fultz Inelastic neutron scattering was used to measure the phonon densities of states (DOS) for pure V and V-6\%X solid solutions, and for the A15 compounds V3Si, V3Ge and V3Co, at temperatures from 10$\,$K to 1300$\,$K. Pure V and the A15 superconductors V3Si and V3Ge exhibit an anomalous anharmonic stiffening of phonons with increasing temperature up to 1000$\,$K. In V, this anharmonicity is suppressed by Co and Pt, but not by iso-electronic Nb solutes. Non-superconducting V3Co exhibits a normal quasi-harmonic softening. The electronic density of states was calculated from first-principles DFT methods for all alloys and compounds. The materials whose phonons behave anomalously also exhibit sharp peaks below the Fermi energy in their electronic DOS. The phonon-enhanced thermal smearing of these sharp features results in reduced screening of nuclear motions and stiffer phonons. These results show that the EPI can influence the phonon thermodynamics at temperatures up to 1000$\,$K. Wednesday, March 12, 2008 12:27PM - 12:39PM Q40.00007: Remarkably large field dependences of the thermodynamic and transport properties of PtSn$_{4}$ P.C. Canfield , S.L. Bud'ko , E.D. Mun , H. Ko , G.D. Samolyuk PtSn$_{4}$ is a known, binary, intermetallic compound that forms as a result of a deeply paritectic reaction. It's reported to have an orthorhombic crystal structure with lattice parameters a = 6.42, b = 11.4, c = 6.39 A. Exceptionally low residual resistivity single crystals of PtSn$_{4}$ have been grown out of excess Sn (with RRR values larger than 1000) and a detailed study of their field dependent properties have been made. The highlights of our results can be summarized as follows: (a) PtSn$_{4}$ manifests a huge, low temperature magnetoresistance of 10$^{5}$ {\%}, for an applied field of 5 T, that rises to 10$^{6}$ {\%} for 14 T; (b) PtSn4 manifests dramatic and clearly resolved oscillations in the magnetization that, for fields below 7T, can be clearly resolved for temperatures as high as 20 K; (c) PtSn$_{4}$ manifests dramatic and clear oscillations in electrical resistivity, that for fields below 14 T, can be clearly resolved for temperatures as high as 10 K. Wednesday, March 12, 2008 12:39PM - 12:51PM Q40.00008: Scanning Tunneling Microscopy Atomic Resolution of Uranium Compound Marilyn Hawley , Shao-Ping Chen , Phillip Van Stockum Room temperature ultra-high vacuum scanning tunneling microscopy (STM) atomic resolution imaging has been achieved for the first time on a layered uranium compound, uranium antimony two. High quality single crystals were cleaved in situ then imaged by STM using PtIr tips. Atomic resolution images revealed an in-plane square lattice with an uranium-uranium interatomic spacing consistent with theoretical predictions for the lowest energy cleavage plane. The STM images revealed a number of, as yet, unexplained features suggestive of missing atoms and single atom wide rows of atoms aligned along the two a-lattice parameter directions, which will be discussed in this talk. Wednesday, March 12, 2008 12:51PM - 1:03PM Q40.00009: Ab initio thermodynamics of LaB$_{6}$ and CeB$_{6}$ Tanju Gurel , Resul Eryigit We have investigated the phonon and thermodynamical properties of rare-earth hexaborides RB$_{6}$ (R=La and Ce) on the basis of first-principles pseudopotential plane wave method together with the quasiharmonic phonon calculations. The phonon dispersion relations which are calculated in the linear response framework and the temperature dependent lattice constant, bulk modulus and the volume expansion coefficients which are calculated in the quasiharmonic approximation are in good agreement with the experimental data. We calculate the low temperature constant volume specific heat due to phonon and electronic degrees of freedom and extract the Kondo contribution to specific heat of CeB$_{6}$. Wednesday, March 12, 2008 1:03PM - 1:15PM Q40.00010: Structure of Boron Carbide: Where's the Carbon? David Marx , Gerald Seidler , Timothy Fister , Kenneth Nagle , Carlo Segre Although the structure of the boron carbide series, B$_{12-x}$C$_{x}$ with 0.06 $\le \quad x \quad \le$ 1.7, has been studied since the 1940s, the location of the carbon atoms has not been adequately determined. The recent development of the lower energy resolution inelastic x-ray scattering (LERIX) spectrometer on the PNC-CAT beamline at the Advanced Photon Source at Argonne National Lab has enabled differentiation of the boron and carbon absorption edge data for the various crystallographic sites. The structure (R-3m) consists of twelve-atom icosahedra and three-atom chains. Boron carbide may have a maximum of three carbon atoms, which may be located on the two end of chain sites and in one of two inequivalent sites on the icosahedra. At least one carbon atom must be present in the structure for it to be stable. In this presentation, structural results from non-resonant x-ray scattering for seven samples, ranging from B$_{4}$C to B$_{10.1}$C will be presented. Wednesday, March 12, 2008 1:15PM - 1:27PM Q40.00011: Electronic structure and properties of lanthanum Lane Nixon , Dimitrios Papaconstantopoulos The total energy and electronic structure of lanthanum have been calculated in the $\it bcc$, $\it fcc$, $\it hcp$ and $\it dhcp$ structures for pressures up to 50 GPa. The full potential linearized-augmented-planewave method was used with both the local-density and general-gradient approximations. The correct phase ordering has been found, with lattice parameters and bulk moduli in good agreement with experimental data. The GGA method shows excellent agreement overall while the LDA results show larger discrepancies. The calculated strain energies for the $\it fcc$ and $\it bcc$ structures demonstrate the respective stable and unstable configurations at ambient conditions. The calculated superconductivity properties under pressure for the $\it fcc$ structure are also found to agree well with measurements. Both LDA and GGA, with minor differences, reproduce well the experimental results for $T_c$. Wednesday, March 12, 2008 1:27PM - 1:39PM Q40.00012: High-Level Correlated Approach to the Jellium Surface Energy, Without Uniform-Electron-Gas Input Lucian Constantin We resolve the long-standing controversy over the surface energy of simple metals: Density functional methods that require uniform-electron-gas input agree with each other at many levels of sophistication, but not with high-level correlated calculations like Fermi Hypernetted Chain and Diffusion Monte Carlo (DMC) that predict the uniform-gas correlation energy. Here we apply a very high-level correlated approach, the inhomogeneous Singwi-Tosi-Land-Sj\"olander (ISTLS) method, and find that the density functionals are indeed reliable (because the surface energy is bulk-like''). ISTLS values are close to recently-revised DMC values. Our work also vindicates the previously-disputed use of uniform-gas-based nonlocal kernels in time-dependent density functional theory. Wednesday, March 12, 2008 1:39PM - 1:51PM Q40.00013: Magnetic, thermodynamic and transport properties of GdFe$_{2}$(Al$_{x}$Zn$_{1-x})_{20}$ M. Lampe , N. Ni , S. Jia , G.D. Samolyuk , A.S. Sefat , S.L. Bud'ko , P.C. Canfield The unusual physical properties of the dilute, rare-earth-bearing, intermetallic compound GdFe$_{2}$Zn$_{20}$ have been explained as being the result of the Gd$^{3+}$ moment being embedded in a nearly ferromagnetic Fermi liquid. To understand this in detail, single crystals of the pseudo ternary series GdFe$_{2}$(Al$_{x}$Zn$_{1-x})_{20}$(x $\le$0.07) and YFe$_{2}$(Al$_{x}$Zn$_{1-x})_{20}^{ }$(x $\le$0.05) were grown out of Zn-rich solution. Magnetization, heat capacity and resistivity measurements on these compounds reveal a decrease of T$_{c}$ from 86 K (x = 0) to 4 K (x = 0.07) for GdFe$_{2}$(Al$_{x}$Zn$_{1-x})_{20}$ and a decrease of the Stoner enhancement factor, Z, from 0.88 (x = 0) to 0.35 (x = 0.05) for YFe$_{2}$(Al$_{x}$Zn$_{1-x})_{20}$. Rigid band approximation and TB-LMTO-ASA calculation are used to explain this trend. These results, combined with earlier studies of the substitution of Co for Fe clearly indicate the importance of band filling and the applicability of even a simple rigid band model, to these compounds. Wednesday, March 12, 2008 1:51PM - 2:03PM Q40.00014: Magnetic properties of RT$_2$Al$_{20}$ (R = Gd, Eu and Yb, T = Ti, V and Cr) J. Frederick , Shuang Jia , S.L. Bud'ko , P.C. Canfield Isostructural RT$_2$Al$_{20}$ series of compounds contain less than 5 at. \% of rare earth ions. Thermodynamic and transport measurements were performed on solution-grown, single crystals: both R = Gd and Eu series manifest clear local moment behavior with magnetic ordering below 10 K. These low transition temperatures are consistent with the dilute nature of the rare earth ions. Unlike the RT$_2$Zn$_{20}$ series, we have not found enhanced magnetic order or near-Stoner like behavior for any member of the RT$_2$Al$_{20}$ family of compounds. The R = Yb members, however, all manifest weak Pauli paramagnetism, consistent with a divalent state for the Yb ions.