Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J20: Insulators: Radiation Detection, Defects, Thermal Properties |
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Sponsoring Units: DMP Chair: Timo Thonhauser, Rutgers University / Massachussetts Institute of Technology Room: Colorado Convention Center 105 |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J20.00001: Substitutional NaCl hydration in ice Peter J. Feibelman Na$^{+}$ and Cl$^{-}$ can replace water molecules in ice Ih, with minimal lattice strain and without disrupting the crystal's H-bond network. \textit{Ab initio} calculations show that substitutional solvation is optimally endothermic by only 0.50 eV per NaCl formula unit. Consistent with Na$^{+}$ and Cl$^{-}$ ionic radii of 1.0 and 1.8 {\AA}, Na$^{+}$ ions in the optimal structure lie 2.43 {\AA} from their four equidistant, nearest O-atom neighbors, and the Cl$^{-}$ ions 3.02 {\AA} from theirs. Solvation of \textit{interstitial} ions is less favorable by at least 1.5 eV. These results are cautionary for molecular dynamics simulations of ionic solvation in ice; a correction for the presence of water interstitials is needed if the number of molecules in the simulation cell is chosen too large, and another for the generation of free Bjerrum defects, if anion and cation solvation are treated separately. [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J20.00002: Investigation of silicon complexes in Si-doped calcium phosphate bioceramics P. Gillespie, M.J. Stott, M. Sayer, G. Wu Silicon doped calcium phosphate materials have drawn great interest as bioceramics for bone repair due to their enhanced bioactivity. However, the low level of doping in these materials, generally $\sim $1 wt.{\%}, makes it difficult to determine the effects the silicon has on the structure of these materials. In this study, silicon substituted hydroxyapatite (Si-HA), silicon stabilized alpha tricalcium phosphate (Si-TCP), and a multi-phase mixture consisting of approximately 75{\%} Si-TCP with the remainder being mainly Si-HA have been synthesized using isotopically enriched silica containing $^{29}$Si. $^{29}$Si magic-angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) has been used to examine the silicon complexes within these materials resulting from the substitution of SiO$_{4}^{4-}$ for PO$_{4}^{3- }$and the required charge compensation mechanism needed to achieve this. Previous ab initio studies on these materials have investigated charge compensation mechanisms to suggest possible silicon complexes and these serve as a basis for interpreting the NMR results. [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J20.00003: Dose-Rate Dependence of Ionizing Radiation Damage in Silicon Transistors Harold Hjalmarson, George Vizkelethy, Charles Hembree, Ronald Pease The primary effects of ionizing radiation on silicon transistors is caused by the effects of electrons and holes created in the oxide portions of these devices. The holes can become trapped in the oxide, and they also create traps at the semiconductor-silicon interface. In the best- accepted explanation, the holes release hydrogen from source sites, often near the interface, and this hydrogen causes the interface traps by reacting with hydrogen-passivated Si dangling bonds at the interface. In this presentation, the transient electrical effects in a silicon device will be computed as a function of radiation duration, total dose, oxide defects, silicon doping and other physical variables. These calculations reveal mechanisms, such as bimolecular defect reactions, that make the damage dependent on the radiation dose rate if the total radiation dose exceeds a threshold dose. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J20.00004: Information-Driven Exploration of Crystal Chemistries for Radiation Detection Materials Kim Ferris, Bobbie-Jo Webb-Robertson, Dumont Jones The ability to suggest promising materials and a priori eliminate unfruitful inquiry is the key to new crystal chemistry searches. Variable spaces tend to be large and poorly defined, and property measurements (and computations) of candidate materials are not abundant. For simple binary systems, the presence of structural polymorphs and higher order compositions for A(m)B(n); m,n=1-3 would combinatorially generate over 300,000 candidates, greatly complicating our ability to explore candidate spaces. We have used knowledge extraction methods to evolve structural signatures to direct searches using performance-based criteria. The exploratory data methods used both supervised (support-vector machines) and unsupervised (disorder-reduction and principal-component) classification methods for structural signature development. The development of new candidates for radiation detection materials will be used as the case example for this talk. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J20.00005: Electronic structure of lead pyrophosphate Malliga Suewattana, David Singh Lead Pyrophosphate Pb$_{2}$P$_{2}$O$_{7}$ is of interest for potential radiation detection applications and use in long term waste storage. It forms in triclinic P${\bar{1}}$ crystals and can also be grown as glasses. We performed electronic structure calculations using the crystal structure which determined by Mullica et. al (J. Solid State Chem (1986)) using x-ray diffraction and found large forces on atoms suggesting that the refined atomic positions were not fully correct. Here we report first principles structure relaxation and a revised crystal structure for this compound. We analyze the resulting structure using pair distribution functions and discuss the implications for the electronic properties. This work was supported by DOE NA22 and the Office of Naval Research. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J20.00006: Full potential LAPW calculations of positron lifetimes in materials Hiroyuki Takenaka, David Singh We report positron lifetime calculations for a large number of semiconductors and insulators, including materials of interest for radiation detection. These include CdTe, ZnTe, lanthanide trihalides, orthophosphates, ZnO. Trends in lifetimes with structural features are discussed. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J20.00007: ABSTRACT HAS BEEN MOVED TO B20.00013 |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J20.00008: Simulated electrolyte-metal interfaces -- Li$_3$PO$_4$ and Li Xiao Xu, Yaojun A. Du, N.A.W. Holzwarth There has recently been a lot of interest in solid electrolyte materials such as LiPON developed at Oak Ridge National Laboratory\footnote{J. B. Bates, N. J. Dudney, and co-workers, {\em{Solid State Ionics}}, {\bf{53-56}}, 647-654 (1992).} for use in Li-ion batteries and other technologies. We report on the results of our model calculations on idealized interfaces between Li$_3$PO$_4$ and Li metal, studying the structural stability and the ion mobility, using first-principles density functional techniques with the {\em{PWscf}} and {\em{pwpaw}} codes.\footnote{http://www.pwscf.org and http://pwpaw.wfu.edu.} Starting with a supercell constructed from Li$_3$PO$_4$ in its crystalline $\gamma$-phase structure and several layers of Li metal, we used optimization and molecular dynamics techniques to find several meta-stable configurations. The qualitative features of the results are consistent with experimental evidence that the electrolyte is quite stable with respect to Li metal.\footnote{N. J. Dudney in Gholam-Abbas Nazri and Gianfranco Pistoia, Eds., {\em{ Lithium Batteries: Science and Technology}}, Chapt. 20, pp. 623$-$642, Kluwer Academic Publishers, 2004. ISBN 1-4020-7628-2.} In addition to stability analyses, we plan to study Li-ion diffusion across the interface. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J20.00009: Li-ion diffusion mechanisms in crystalline Li$_3$PO$_4$ electrolytes Yaojun A. Du, N.A.W. Holzwarth Using first principles electronic structure methods and ``nudged elastic band'' optimization techniques,\footnote{Using the Quantum ESPRESSO package http://www.pwscf.org/.} we examine ideal Li-ion diffusion in crystalline Li$_3$PO$_4$ electrolytes, considering both vacancy and interstitial mechanisms. The simulations are performed in supercells containing 16 Li$_3$PO$_4$ units. We determine the activation barriers for several plausible diffusion paths, considering the effects of the exchange-correlation functional forms, of the crystalline form in the $\beta$- and $\gamma$- structures, and also the effects of substitutional N. Using the generalized gradient approximation, results for $\gamma$-Li$_3$PO$_4$ show diffusion barriers of 0.6-0.7~eV for the vacancy mechanism with a small dependence on the crystallographic direction. For the interstitial mechanism, the diffusion barriers are 0.8~eV and 1.3~eV along the ${\bf{b}}$- and ${\bf{c}}$-axes, respectively. The larger activation barriers of the interstitial mechanism are closer to experimental measurements on polycrystalline and single crystal samples\footnote{J. Solid St. Chem. {\bf{115}}, 313 (1995), Cryst. Rep. {\bf{46}}, 864 (2001) } which find $E_a \approx 1.1-1.3$~eV. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J20.00010: Negative-thermal-expansion ZrW$_{2}$O$_{8}$. Elasticity and pressure Cristian Pantea, Albert Migliori, Peter Littlewood, Yusheng Zhao, Hassel Ledbetter, Jason Lashley, Tsuyoshi Kimura, Joost Van Duijn, Glen Kowach The elasticity of the negative thermal expansion (NTE) compound ZrW$_{2}$O$_{8}$ is rather strange: the solid softens as its volume decreases on warming. Does ZrW$_{2}$O$_{8}$ also soften when pressure alone is applied? Using pulse-echo ultrasound in a large-volume moissanite anvil cell, we find an unusual decrease in bulk modulus with pressure at 300K. Our results are inconsistent with conventional lattice dynamics, but a framework-solid-based non-linear model with many degrees of freedom predicts elastic softening as increases in either temperature or pressure reduce volume. The pressure-induced phase transition from $\alpha $-ZrW$_{2}$O$_{8}$ (cubic) to $\gamma $-ZrW$_{2}$O$_{8}$ (orthorhombic) is found to take place at $\approx $ 0.5 GPa, result confirmed by Raman spectroscopy. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J20.00011: Specific heat of tri-glycine sulfate in electric field Jason Lashley, N. Hur, Mike Hundley, W. Hults, Jason Cooley, Bogdan Mihaila, James Smith, Trevor Finlayson, Cyril Opeil, Robert Fisher Measurements of the specific heat, polarization and dielectric constant are reported in electric fields up to 3 kV/cm in order to investigate domain energetics in the hydrogen-bonded ferroelectric tri-glycine sulfate. Although the shape of the specific-heat anomaly at $T_C = 322.5\, K$ is thermally broadened in unpoled crystals, this shape changes into the characteristic $\lambda$-shape expected for a continuous transition with the application of a 220$\,$V/cm electric field. The $\lambda$ transition below $T_C$ depends on $T-T_C$ in a range $T-T_c\leq~10~K$ with a critical exponent, $\beta$ = - 0.39. Similarly we find that below $T_C$ the experimental dielectric constant obeys an inverse-power law of the form, $\varepsilon(T) = a/(T-T_C)^\beta$, with the constant $a = 1244\, \mathrm{K}$ and the exponent $\beta=0.4$. [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J20.00012: Effect of temperature on the thermodynamic properties of Na$_2$Ti$_3$O$_7$ Hong Zhang, Haiying Wu The equilibrium structure of the compound Na$_2$Ti$_3$O$_7$ has been obtained via the minimization of the total energy within Local Density Approximation (LDA) based on Density Functional Theory (DFT), the calculated equilibrium volume are in agreement with available experimental values. In the meantime, the thermodynamic properties are investigated applying nonempirical Debye-like model combining with the first principle theory in the quasi-harmonic approximation. The evaluated equilibrium volume using this model agrees with the values from \textit{ab intio} and from experiment. Our results demonstrate that this method can provide reliable predictions for the temperature dependence of these quantities such as the equation of state, the bulk modulus, the heat capacity, and the thermal expansion in detail. And our calculated thermodynamic properties are all in agreement with available experimental data. [Preview Abstract] |
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