Bulletin of the American Physical Society
2008 Joint Fall Meeting of the Texas and Four Corners Sections of APS, AAPT, and Zones 13 and 16 of SPS, and the Societies of Hispanic & Black Physicists
Volume 53, Number 11
Friday–Saturday, October 17–18, 2008; El Paso, Texas
Session H6: Diffraction and Scattering |
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Chair: Miguel Castro-Colin, The University of Texas at El Paso Room: Union East, 3rd Floor Wiggins |
Saturday, October 18, 2008 10:30AM - 10:42AM |
H6.00001: Neutron Diffraction Studies of Hydrogen Adsorption in a Highly Stable Porous Rare-Earth Metal-Organic Framework Junhua Luo, Yusheng Zhao, Hongwu Xu, Luc L. Daemen Gas sorption measurements show that a highly stable porous lanthanide metal-organic framework can take up hydrogen of about 2.1 wt. {\%} at 77 K and 10 bar. Difference Fourier analysis of neutron powder diffraction data revealed four distinct D$_{2}$ sites that are progressively filled within the nanoporous framework. Interestingly, the strongest adsorption sites identified are associated with the aromatic organic linkers rather than the open metal sites, as occurred in previously reported MOFs. Our results provide for the first time direct structural evidence demonstrating that optimal pore size (around 6 {\AA}, twice the kinetic diameter of hydrogen) strengthens the interactions between H$_{2}$ molecules and pore walls and increases the heat of adsorption, which thus allows for enhancing hydrogen adsorption from the interaction between hydrogen molecules with the pore walls rather than with the normally stronger adsorption sites (the open metal sites) within the framework. At high concentration H2-loadings (5.5 H2 molecules (3.7 wt. {\%}) per Y(BTC) formula), H2 molecules form highly symmetric novel nanoclusters with relatively short H2-H2 distances compared to solid H2. These observations are important and hold the key to optimizing this new class of rare metal-organic frameworks (RMOFs) materials for practical hydrogen storage applications. [Preview Abstract] |
Saturday, October 18, 2008 10:42AM - 10:54AM |
H6.00002: Evolution of Magnetism in Sr$_{2-x}$La$_{x}$FeO$_{4 }$ system of compounds at low temperature Karunakar Kothapalli, Abdel Alsmadi, H. Kawanaka, Heinz Nakotte We report on the evolution of magnetism at low temperature in two compounds of the Sr$_{2-x}$La$_{x}$FeO$_{4}$ system. Our studies confirmed the room temperature structures of both the compounds that crystallize in the tetragonal K$_{2}$NiF$_{4}$ structure with I4/mmm space group. Both the compounds have antiferromagnetic ordering temperatures above 100K. Low temperature neutron diffraction were taken on the Single Crystal Diffractometer at Los Alamos Neutron Science Center and Intensed Pulsed Neutron Source(IPNS) to gain more insight into the evolution of magnetism below 30K and the nature of transitions evidenced in the previous studies. [Preview Abstract] |
Saturday, October 18, 2008 10:54AM - 11:06AM |
H6.00003: Total electron scattering cross section of Fluorocarbons at intermediate electron energies Prasanga Palihawadana, Gilberto Villela, Wickramasinghe Ariyasinghe Total electron scattering cross sections (TCS) of Tetrafluoromethane (CF$_{4})$, Trifluoromethane (CHF$_{3})$, Hexafluoroethane (C$_{2}$F$_{6})$ and Octafluorocyclobutane (C$_{4}$F$_{8})$ have been measured using the linear transmission technique for impact energies 0.10 -- 4.00 keV. These TCS are compared to existing experimental and theoretical TCS in the literature. Based on the present measurements, an empirical formula is developed to predict the TCS of fluorocarbons as a function of incident electron energy. [Preview Abstract] |
Saturday, October 18, 2008 11:06AM - 11:18AM |
H6.00004: Negative thermal expansion in hexacyanocobaltates of divalent metals Sourav Adak, Luke Daemen, Heinrich Nakotte While many Prussian Blue analogs are known to display negative thermal expansion (NTE), few have been studied in detail. Not all compounds in this family exhibit NTE. Because it is possible to systematically vary ion size and charge in these materials, they represent an interesting playground to study NTE and possible correlations with electronic and crystal structures. By contrast with many silicates displaying NTE and in which tetrahedral units are linked with apical oxygens, the octahedral units in Prussian Blue compounds are linked with a linear cyanide ligand. This introduces more degrees of freedom in the (mostly) cubic structures. Powder samples of hexacyanocobaltates of divalent metals with general formula$M_3^{II} [Co^{III}(CN)_6 ]_2 $ (M = Co, Cu, Mn, Ni, and Zn) were prepared via standard chemical precipitation. X-ray powder diffraction patterns were collected at room temperature (T$_{2 }$= 300K) and at liquid nitrogen temperature (T$_{1}$ = 77K) to obtain an average coefficient of thermal expansion. The X-ray data were analyzed using the Rietveld refinement technique with the General Structure Analysis System (GSAS) software. All the materials studied are cubic with space group $Fm\overline 3 m$ or $F\overline 4 3m$. The thermal expansion coefficients of the five compounds are determined to be in the range 3.5 x 10$^{-6 }$K$^{-1}$ -- 46.6 x 10$^{-6}$ K$^{-1}$. [Preview Abstract] |
Saturday, October 18, 2008 11:18AM - 11:30AM |
H6.00005: Fabrication and Optical Intensity Analysis of Nd$^{3+}$:Y$_{2}$O$_{3}$ nanoparticles embedded in PMMA Sreerenjini Chandra, John B. Gruber, Dhiraj K. Sardar The fabrication, structural morphology, and optical characterization of Nd$^{3+}$:Y$_{2}$O$_{3}$ ions doped in the plastic, Polymethyl Methacrylate (PMMA) will be discussed. The Judd-Ofelt intensity parameters obtained by analyzing the room temperature absorption spectrum of the sample are used to determine the radiative decay rates and branching ratios of different transitions from the upper manifold to the lower lying manifolds. Using the radiative decay rate values, radiative lifetimes of the excited states have been determined. We report the low-temperature absorption spectra of Nd$^{3+}$:Y$_{2}$O$_{3}$/PMMA nanocomposites. A modeling analysis based on the Monte-Carlo method was used to determine the atomic and crystal-field parameters corresponding to the best (global) minimum between theoretical eigenvalues and experimental Stark sublevels. The room temperature fluorescence lifetimes were measured for intermanifold emission transitions. The detailed study reveals the potential of Nd$^{3+}$:Y$_{2}$O$_{3}$/ PMMA nanocomposites as an efficient laser host. [Preview Abstract] |
Saturday, October 18, 2008 11:30AM - 11:42AM |
H6.00006: Atom-interferomtery measurements of dispersive phase shifts for matter waves due to nano-structures Vincent P.A. Lonij, Alexander D. Cronin, Steven Lepoutre, Haikel Jelassi, G\'erard Tr\'enec, Matthias B\"uchner, Jacques Vigu\'e Atom interferometers are renowned for their ability to measure phase shifts with great precision. We have improved an atom interferometer experiment to measure phase shifts due to Van der Waals atom-surface interactions with enough precision to detect an unusual velocity dependence. We used standing waves of light, as beam splitters, and a nano-fabricated structure as a phase shifter. One arm of the Mach-Zehnder atom interferometer was transmitted through a nano-structure with 50 nm wide openings. This nano-structure exhibits a refractive index for matter-waves with a dispersion relation that is different from other, previously known, dispersive elements in atom-optics, such as electric fields or dilute gasses. The phase shifts for lithium atoms in the velocity range 700 to 2000 m/s depends on velocity to the power -0.47 (whereas for electric fields the phase shift depends on velocity to the power -1, and a dilute gas causes phase shifts that undulate as a function of velocity). We discuss the origin of this newly observed velocity dependence, and the utility of this experiment to measure Van der Waals $C_3$ coefficients. [Preview Abstract] |
Saturday, October 18, 2008 11:42AM - 11:54AM |
H6.00007: Semi-Quantitative Analysis of Standard Reference Materials (SRM) and Metallic Plates through X-ray Fluorescence using a Portable Pyro-electric X-ray Generator. Sunil Kumar Valaparla, Nick del Rio We present results obtained during irradiation with a pyro-electric X-ray generator of metallic samples: 1240c aluminium alloy, 1159 electronic and magnetic alloy, and C2417 lead-base alloy. The spectra were analyzed using a CdTe and Si (Li) energy-dispersive detectors connected to a multichannel analyzer; all assembled in a portable configuration. Energy efficiency calibration of the detectors used was performed to produce quantitative results about the elemental composition of the samples. The results obtained are comparable to those reported by other methods. [Preview Abstract] |
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