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 B4: Crystal Lattices: Experiment and Computation |
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Chair: Dieter Hochheimer, Colorado State University Room: Union East, 3rd Floor Elkins |
Friday, October 17, 2008 10:30AM - 10:42AM |
B4.00001: Searching for Natures Missing Crystal Structure Thomas O. McConkie, David D. Allred, Gus L.W. Hart, Brian Hicks A recent report in Nature Materials inferred the existence of a few binary crystal structures never before observed or created. The crystal structures are simple derivative superstructures of the fcc lattice, with only 4 atoms/cell. One of these crystal structures, dubbed L1$_{3}$ by the author, has been predicted to exist in several binary intermetallic systems. To test these predictions, we used sputtering techniques to fabricate thin samples of Pt-Cd close to 3:1 stoichiometry. We analyzed the samples using electron diffraction and X-ray diffraction. We have found evidence of a new phase but are yet unable to confirm the exact ordering. [Preview Abstract] |
Friday, October 17, 2008 10:42AM - 10:54AM |
B4.00002: Single x-ray photon diffraction Alexander Panin Single photon diffraction in visible light is very difficult to demonstrate as it requires very low intensity sources and very sensitive detectors. In x-ray range of electromagnetic spectrum these conditions are much more relaxed due to much higher probability of spontaneous emission and much higher energy of photons, so experiments with single photon diffraction can be easily arranged with ordinary x-ray sources (like x-ray tube) and ordinary detectors (like Geiger counter, CCD array, etc). We have performed such experiments. Analytical comparison of single photon x-ray diffraction and visible light diffraction, and the discussion of results of our experiment are presented in the talk. [Preview Abstract] |
Friday, October 17, 2008 10:54AM - 11:06AM |
B4.00003: High-Temperature Phase Transitions in CsH$_{2}$PO$_{4}$ Cristian Botez, Juan Hermosillo, Jianzhong Zhang, Jiang Qian, Yusheng Zhao, Juraj Majzlan, Russell Chianelli In order to uncover the microscopic origin of the temperature-induced three-order-of-magnitude jump in the proton conductivity of CsH$_{2}$PO$_{4}$ (superprotonic behavior), its crystal structure modifications within the 25\r{ }C -300\r{ }C temperature range under both ambient- and high-pressure conditions have been investigated using synchrotron X-ray diffraction. The high-pressure data show no indication of the thermal decomposition/polymerization at the crystal surface recently proposed as the origin of the enhanced proton conductivity [Phys. Rev B \textbf{79}, 054104 (2004)]. Instead, direct evidence that the superprotonic behavior of the title material is associated with a polymorphic structural transition to a high-temperature cubic phase has been found. [Preview Abstract] |
Friday, October 17, 2008 11:06AM - 11:18AM |
B4.00004: Finding new structures in hexagonal-close-packed alloys Gus Hart, Rodney Forcade Because of their exceptional strength-to-weight ratios, magnesium-based alloys could play a critical role in increasing the fuel efficiency of automobiles. But much of the materials science of magnesium alloys is unknown. One particularly important question is how to improve the creep resistance of the alloys by precipitate hardening. Finding new compounds and structures that form in magnesium alloys could provide the key to developing a new material. We have developed several new approaches to explore all possible structures in hexagonal-close-packed systems (such as magnesium) and determine which are promising. [Preview Abstract] |
Friday, October 17, 2008 11:18AM - 11:30AM |
B4.00005: Predicting new structures in B-cation ordering perovskites Matthew Lords, Gus Hart Material properties are intimately tied to crystal structure. Many materials, alloys in particular, share a common, underlying ``motif'', such as an fcc/bcc/hcp ``parent lattice,'' but have different chemical orderings. Among the infinite possibilities for chemical orderings, why does nature choose the few it does? We answer this question generally by using a method that only looks at the geometry of the structure. Using this structure-analysis method we examine possible new ordering for a class of simple cubic structures, the ever-important perovskites. [Preview Abstract] |
Friday, October 17, 2008 11:30AM - 11:42AM |
B4.00006: Calculation of NMR lineshapes for Ba-Al-Ge clathrates Sergio Rodriguez, Weiping Gou, Joseph Ross Clathrates consist of Si, Ge, or Sn cages in a crystalline framework, with guest atoms inside the cages. They have gained interest due to thermoelectric properties suitable for potential device application. To understand Al substitutional configurations, we calculated Al NMR line shapes for several structures with compositions Ba$_{8}$Ge$_{46-x-y}$Al$_{x}\Box_y$ for $x=3,8,11,12,16,24$; $y=2,3$; where $\Box$ represents a vacancy. The results were obtained by calculating Electric Field Gradients (EFG) for Al sites of type-I clathrates assuming an ordered superstructure of vacancies and framework occupation. We used $ab$ $initio$ methods in the Generalized Gradient Approximation as implemented by the WIEN2k program, and used the results to simulate NMR lineshapes numerically. These were compared to our previously reported NMR lineshapes. In the case of Ba$_{8}$Ge$_{31}$Al$_{12}\Box _3$ four Al sites in the superstructure include two sites with small EFG where the vacancy is far away and two sites with large EFG with a vacancy adjacent to Al. Assuming a larger Knight shift for sites next to vacancies, we obtain good agreement with NMR experimental results for reduced-Al Ba$_{8}$Ge$_{34}$Al$_{12}$, while for the Zintl phase Ba$_{8}$Ge$_{30}$Al$_{16}$ we obtain good agreement with no spontaneous vacancies. We infer that Al prefers locations close to vacancies rather than random occupation. This work was supported by Robert A. Welch Foundation (Grant A-1526). [Preview Abstract] |
Friday, October 17, 2008 11:42AM - 11:54AM |
B4.00007: New Structures for Jewelry Applications in Palladium Jacqueline Corbitt, Erin Gilmartin, Gus Hart An intriguing intermetallic structure with 8:1 stoichiometry was discovered in the 1950s in the Pt-Ti system. Since then a handful of other Pt/Pd/Ni binary systems have been observed to exhibit this curious structure (Pt$_8$Zr, Pd$_8$Mo, Ni$_8$Nb, etc). This ordered structure can significantly increase the hardness of the material. For jewelry applications involving Pt and Pd, international hallmarking standards require that the alloys be at least 95\% pure by weight. However, these metals are often soft when purity is high if the minority atoms are disordered. Because the 8:1 structure maintains a high weight percentage of Pt/Pd, it can satisfy purity standards while increasing performance. Recent calculations suggest that the 8:1 structure may form in about 20 previously unsuspected Pt/Pd binary systems. For this study, we investigated the possibility of Pd-rich compounds in Pd-Nb and Pd-Cu. [Preview Abstract] |
Friday, October 17, 2008 11:54AM - 12:06PM |
B4.00008: Alternative alloys for platinum jewelry? New structures in Pt-Hf and Pt-Mo Erin Gilmartin, Jacqueline Corbitt, Gus Hart The only known intermetallic structure with an 8:1 stoichiometry is that of Pt$_8$Ti. It is intriguing that an ordered compound would occur at such low concentrations of the minority atom. But this structure occurs in about a dozen binary intermetallic systems. The formation of an ordered structure can significantly enhance the performance of the material, particularly the hardness. Pt- and Pd-rich ordered structures have been experimentally studied in the systems Pt/Pd-X where X is Ti, V, Cr, Zr, Nb, M, Hf, Ta, and W. We took a broader look at 80 Pt/Pd rich alloys to find new candidates for the 8:1 structure and have found about 20. In order to verify our predictions, we used the cluster expansion to find the stable structures. We first applied the cluster expansion to Pt-Hf and Pt-Mo because these two candidates are the most likely to form the 8:1 structure. These new candidates can have applications in the jewelry and catalysis industries. [Preview Abstract] |
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