Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y27: Focus Session: Chemical Control of the Properties of Complex Oxides II |
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Sponsoring Units: DCP Chair: Ram Seshadri, University of California, Santa Barbara Room: D137 |
Friday, March 19, 2010 8:00AM - 8:36AM |
Y27.00001: Chemical control of the structures and properties of layered oxide chalcogenides and oxide pnictides Invited Speaker: Mixed anion compounds composed of oxide layers separated by chalcogenide or pnictide layers have diverse chemical, electronic and magnetic properties. The recently discovered layered iron oxide pnictides and related materials are of particular interest as they display unconventional superconductivity. In this lecture the chemical control of the composition of layered compounds in which transition metal oxide perovskite-type layers are separated by copper or silver chalcogenide layers will be described. The focus will be on the use of low temperature, ``soft'' chemical techniques to tune the composition. The correlations between composition, features of the crystal structures, and physical properties such as magnetic order will be described for several related systems. Comparison will be made between these layered oxide chalcogenides and analogous layered iron pnictide superconductors in which the occurrence of superconductivity is often sharply dependent on the composition. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y27.00002: New Polar Oxide Materials: Design Strategies and Functional Properties P. Shiv Halasyamani, Hong-Young Chang, Sang-Hwan Kim, Kang Min Ok Polar oxide materials -- those with a macroscopic dipole moment -- are ubiquitous in advanced technologies, i.e. computer memories, sensors, etc. The design and synthesis of new polar oxides remains an ongoing challenge. Macroscopic polarity implies microscopic polarity, in other words some of the coordination polyhedra must be polar. Frequently, the local dipole moment in the coordination polyhedra is directed in opposite directions resulting in a non-polar material. In this presentation we will discuss strategies toward designing new polar oxide materials, as well as the characterization of their functional properties. Specifically we will demonstrate that by using cations susceptible to second-order Jahn-Teller (SOJT) effects we can substantially increase the incidence of macroscopic polarity in any new material. We will describe the synthesis of characterization of several new polar materials, as well as their functional properties and structure-property relationships. [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:24AM |
Y27.00003: Dielectric oxides: How to enhance their beneficial properties Invited Speaker: Dielectric materials are essential for a large number of applications. It is extensively used for electrical energy storage in capacitors and also plays a crucial role in miniaturization of MOSFET's. However, further shrinking of the physical size of such devices with same device capabilities will require the discovery of materials with higher dielectric constants and lower losses than available at present. In fact, in most cases, a combination of several dielectric properties is considered to be optimal, for example, minimal temperature and frequency dependencies of dielectric properties being important in many cases in addition to a large value of the dielectric constant and a low loss. In this talk, I shall discuss two separate series of oxides with useful dielectric properties. In the first example, I shall show that a disorder in the B-site of a family of disordered hexagonal transition metal oxides leads to robust and unusually enhanced dielectric properties, suggesting this to be a general route to prepare oxide materials with such advance properties. In the second example, we start with SrTiO$_{3}$ which is a well-known dielectric material. We shall then discuss various possibilities to enhance dielectric properties of SrTiO$_{3}$ by suitable doping. This work is based on collaborative studies involving the following people: Debraj Choudhury, S. V. Bhat, K. T. Delaney, J. Gopalakrishnan, C. Kakarla, Olof Karis, P. Lazor, R. Mathieu, P. Mondal, , P. Nordblad, A. Negi, R.Nirmala, B. Sanyal, N. A. Spaldin, P. Sujatha Devi, A. Sundaresan, A. Venimadhav, U. V. Waghmare and D. D. Sarma [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y27.00004: Local structure of the lone-pair pyrochlore Bi$_2$Ti$_2$O$_7$ and the search for polar ordering Daniel Shoemaker, Ram Seshadri, Andrew Hector, Anna Llobet, Thomas Proffen Pyrochlore oxides of the formula $A_2B_2$O$_7$ contain geometrically frustrated $A$ and $B$ sublattices, leading to a multitude of complex phenomena including high dielectric constants, glassy spin interactions, and low-temperature peaks in heat capacity. In Bi$_2$Ti$_2$O$_7$, large atomic displacements ($\sim 0.4$ \AA) on the diamond-type OBi$_4$ sublattice cannot cooperatively order to accommodate the Bi lone pair. Instead, polar distortions form a charge ice with no long-range order, and powder diffraction finds Bi$_2$Ti$_2$O$_7$ to remain centrosymmetric at 2 K. We move beyond Rietveld analysis to describe the real-space, local structure of this highly disordered oxide. By conducting large-box reverse Monte Carlo (RMC) simulations on neutron total scattering data, we produce a model that contains details invisible to traditional crystallographic techniques. In addition to describing the local structure of Bi$_2$Ti$_2$O$_7$, we present capabilities of the RMC technique and its application to complex disorder in other crystalline materials. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y27.00005: High-pressure synthesis, crystal and electronic structures of a new scandium tungstate Sc$_{0.67}$WO$_{4}$ John Mitchell, Tamas Varga, Christo Malliakas, Brian Toby, Jun Wang, Lindsay Arnold Negative thermal expansion (NTE) materials possess a low-density, open structure that can respond to high pressure conditions, with potential for new compounds with unusual physical properties. Here we report that one such NTE material---white, insulating, orthorhombic Sc$_{2}$W$_{3}$O$_{12}$---transforms into a black, electrically conducting compound when treated at 4 GPa and 1400 $^{\circ}$C. The high pressure phase, Sc$_{0.67}$WO$_{4}$, crystallizes in a defect-rich wolframite-type structure, a dense, monoclinic structure (space group $P12/c1)$ containing 1-D chains of edge-sharing WO$_{6}$ octahedra.. The chemical bonding of Sc$_{0.67}$WO$_{4}$ vis-\`{a}-vis the ambient pressure Sc$_{2}$W$_{3}$O$_{12}$ phase can be understood on the basis of the Sc defect structure. Magnetic susceptibility, electrical conductivity, and thermoelectric power measurements reveal that Sc$_{0.67}$WO$_{4}$ is a metallic paramagnet. Conductivity varies linearly with temperature from 3-300 K, which may be understood in terms of weak localization and electron-electron interactions in this poor metal. Oxygen vacancies are suggested as a potential mechanism for generating the carriers in this defective wolframite. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:24AM |
Y27.00006: Discovery of a universal morphotropic phase boundary behavior in rare-earth substituted BiFeO$_{3}$ using thin film composition spreads Invited Speaker: Epitaxial thin film composition spreads can be used to perform systematic substitution and chemical modification studies where changes in structural and physical properties of materials can be continuously tracked. I will discuss one particular example where we study substitution of various trivalent RE$^{ }$ions into the A-site of BiFeO$_{3}$ (BFO) using a series of composition spreads. We had earlier reported on the transition of the rhombohedral ferroelectric structure of the undoped BFO to an orthorhombic phase in Sm-substituted BFO [S. Fujino et al., Appl.Phys.Lett. 92, 202904 (2008)]. At the structural phase boundary, electromechanical properties including the piezoelectric coefficient d$_{33}$ and the dielectric constant are substantially enhanced. The value of d$_{33}$ at the boundary can be as high as 110 pm/V, nearly double that of values typically reported for undoped BFO thin films. We show that there is a universal behavior with all RE dopants and that the structural transition accompanied by enhanced dielectric and piezoelectric properties can be universally described by one control parameter, the A-site average ionic radius. This work is performed in collaboration with D. Kan, L. P\'{a}lov\'{a}, V. Anbusathaiah, C.-J. Cheng, S. Fujino, V. Nagarajan, and K. M. Rabe. [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y27.00007: Observations and modeling of defect structures in non-stoichiometric SrTiO$_{3}$ thin films N. Antolin, R. Mishra, O.D. Restrepo, M. Dixit, R. Srinivasan, R. Pond, W. Windl, L. Brillson, H. Fraser Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in thin films of Sr$_{1+\delta }$Ti$_{1-\delta }$O$_{3-\delta }$ ($\delta \sim $0.2) grown on stoichiometric SrTiO$_{3}$. We envision the creation of regularly spaced anti-phase domains (APD's) separated by anti-phase boundaries (APB's) through a a/2$<$111$>$ displacement, thus enabling a locally charge-neutral SrO stoichiometry at the APB's, while preserving the SrTiO$_{3}$ stoichiometry in the interior of the APD's. This allows for an overall charge-neutrality and correct stoichiometry in the non-stoichiometric layer. Preliminary calculations of the dimensions of the APD's are in good agreement with the observed results. We performed molecular dynamics simulations using Buckingham plus Coulomb empirical potentials and their energetics as well as the equilibrium positions of the atoms and resulting lattice constants were determined. Also, the corresponding STEM signals of the structures were modeled, allowing direct comparison to the STEM images of the non-stoichiometric material. To examine the stability of the observed structures in comparison to the constitutional point defects, we calculated the formation energies of single point defects in SrTiO$_{3}$, which we also used to benchmark the empirical-potential results against first-principles values. [Preview Abstract] |
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