Bulletin of the American Physical Society
Fall 2009 Meeting of the Four Corners Section of the APS
Volume 54, Number 14
Friday–Saturday, October 23–24, 2009; Golden, Colorado
Session F7: Materials Physics I |
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Chair: Zhibin Lin, Colorado School of Mines Room: Hill Hall 202 |
Saturday, October 24, 2009 8:00AM - 8:24AM |
F7.00001: Finding one of Nature's missing crystal structures Invited Speaker: There are an infinite number of ways atoms can be arranged on a lattice. But, like the integers, the arrangements can be counted and some are ``smaller'' than others. These small crystal structures are popular in common alloys. But mother Nature refuses to use some other small crystal structures. Why? Maybe she really uses them and we just haven't looked in the right places. We have developed an algorithm for generating a complete list of the possible structures. Using a combination of computation and experiment, we are closing in on a never-before-observed crystal structure. [Preview Abstract] |
Saturday, October 24, 2009 8:24AM - 8:36AM |
F7.00002: Electric-field dependent measurements of 3D x-ray diffuse scattering in piezoelectric materials Benjamin Frandsen, Stacey Smith, Branton J. Campbell, Matthew J. Gardner, Kevin D. Seppi Polar nano-regions (PNR) in relaxor materials Pb(Zn$_{1/3}$Nb$_{2/3})$O$_{3}$ and Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$ are of pressing applied interest due to their influence on the remarkable piezoelectric properties of their solid solutions with ferroelectric PbTiO$_{3}$. ~X-ray single-crystal diffuse-scattering techniques have recently been shown to provide qualitative insight into the local atomic structures of these materials. ~But in order to differentiate among the PNR models that have arisen, quantitative analyses are also needed, which require mapping out large high-precision volumes of reciprocal space as a function of electric field. ~In August of this year, we conducted a synchrotron x-ray diffuse scattering experiment at the Advanced Photon Source at Argonne National Laboratory to determine the effect of a strong electric field on the local atomic structure of a PNR. I will describe the preliminary results of this experiment. [Preview Abstract] |
Saturday, October 24, 2009 8:36AM - 8:48AM |
F7.00003: Negative thermal expansion in hexacyanoferrates(III) of divalent metals S. Adak, L. Daemen, H. Nakotte, D. Williams Many Prussian Blue (PB) analogs are known to exhibit negative thermal expansion (NTE) behavior. However, detail studies of the NTE behavior in these compounds and the underlying mechanism behind such behavior are often missing in the literature. It is possible to systematically vary the charge and the ion size in the PB analogs. The octahedral units in PB analogs are linked with a linear cyanide ligand. This introduces more degrees of freedom in the (mostly) cubic structures. Therefore, the PB compounds offer an interesting playground to study NTE phenomenon and its possible correlations with crystal and electronic structures. The thermal expansion behavior of five PB analogs, hexacyanoferrates(III) of divalent metals with general formula$M_3^{II} [Fe^{III}(CN)_6 ]_2 $ (M = Mn, Co, Ni, Cu, and Zn), has been studied using X-ray powder diffraction measurements. Polycrystalline samples of the studied compounds were prepared via standard chemical precipitation. The X-ray data collected at 300 and 84 K while cooling were analyzed using the Rietveld refinement technique. The crystal structures of the materials studied are cubic with space group$Fm\overline 3 m$ or $F\overline 4 3m$. The analysis indicates the occurrence of NTE in hexacyanoferrates(III) of Co, Cu, and Zn. The NTE coefficients were found to be in the range 15 x 10$^{-6}$K$^{-1}$ -- 31 x 10$^{-6}$ K$^{-1}$. The other two compounds exhibit positive thermal expansion. [Preview Abstract] |
Saturday, October 24, 2009 8:48AM - 9:00AM |
F7.00004: Trends in core-level shifts at bimetallic interfaces formed by group-10 metals deposited on W(110) D.M. Riffe The monolayer bimetallic interface formed by the deposition of Ni, Pd, or Pt on W(110) provides a quintessential setting for investigating the chemical interactions between early and late transition metals. Perhaps surprisingly, these group-10 metals behave as noble metals in this setting. To study the chemical interactions in detail we have obtained core-level photoemission data from W(110) surface atoms for group-10 metals deposited at submonolayer to monolayer coverages. Commonalities among the bimetallic interfaces include the following: (i) a separate substrate core-level shift can be identified for each overlayer phase (1D, pseudomorphic, and/or commensurate); (ii) commensurate overlayers produce only one substrate core-level shift, even though not all substrate atoms are equivalently coordinated; (iii) the difference in substrate shifts induced by pseudomorphic and commensurate overlayers contains a large structural contribution; (iv) for a pseudomorphic overlayer the ratio of the group-10-atom to W-atom core-level shift (when referenced to binding energies at the respective clean surfaces) is ~4 for all three systems; (v) a partial-shift Born-Haber-cycle analysis semiquantitatively describes the substrate shifts induced by both the pseudomorphic and commensurate layers; and (vi) the corresponding core-level shifts on W(110) are very similar to those induced on W(111). [Preview Abstract] |
Saturday, October 24, 2009 9:00AM - 9:12AM |
F7.00005: Ordered Magnesium-Lithium alloys Richard Taylor, Stefano Curtarolo, Gus Hart Emerging technologies increasingly depend on the production of ultra-lightweight materials. Magnesium-lithium (MgLi) alloys are the lightest metallic alloys, having densities near that of plastics, and are strong enough to be used in a variety of high- performance applications. Although considerable work has been done on the MgLi system, little is known regarding potential ordered phases. An analysis of the system with first-principles methods revealed an unexpected wealth of stable zero- temperature phases. Of particular practical interest are configurations containing more than 13 atomic percent lithium, as they will be more ductile due to partial or complete formation on a cubic lattice. The analysis was extended to finite temperature using a Monte Carlo algorithm on large lattices with periodic boundary conditions. Discontinuities in specific heat measurements revealed order-disorder transition temperatures in the range 200-400K. Given the comparatively low melting point of Li ($\sim$450K), kinetics at these temperatures may be sufficient to permit spontaneous partial ordering for Li rich alloys. [Preview Abstract] |
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