Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B38: Structure, Electronic Properties, Spectroscopy, and Ionic Transport in Insulators |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Sudakar Chandran, Wayne State University Room: Morial Convention Center 230 |
Monday, March 10, 2008 11:15AM - 11:27AM |
B38.00001: Perturbed Angular Correlation Study of ZrSiO$_{4}$ and HfSiO$_{4}$ Herbert Jaeger, Sean McBride Time-differential perturbed angular correlation spectroscopy is a powerful technique to study short-range interactions between probe nuclei and host crystals. The technique requires a very low concentration of radioactive probe nuclei and relies on the probes substituting for atoms of the host crystal. We report on PAC measurements of synthetic zircon and hafnon using $^{181}$Ta-probes to study the EFG at Zr and Hf lattice-sites. The quadrupole coupling constants for both zircon and hafnon decrease linearly with increasing temperature. The slope of $\nu _{Q}$ vs. T increases above 800$^{\circ}$C (zircon) and 1100$^{\circ}$C (hafnon) as a consequence of a change in the Zr-O and Hf-O coordination. We also observe a systematic reduction of the anisotropy value $\vert$A$_{2}$$\vert$ that is correlated with the onset of the structural change. Changes in the anisotropy are usually related to dynamic interaction and can be due to nuclear decay after effects. In our case we believe that we see evidence of a trapped electronic defect, giving rise to a rapidly decaying high-frequency quadrupole interaction. At higher temperatures the defect detraps and no longer causes a reduction in the anisotropy. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B38.00002: Stability and lattice dynamics of SiO$_2$ cristobalite Sinisa Coh, David Vanderbilt Among the phases of SiO$_2$ are alpha and beta cristobalite. Despite early indications that the higher-temperature beta phase might be cubic (Fd$\overline{3}$m), it is now accepted that it is in fact tetragonal (I$\overline{4}$2d), and that the experiments suggesting a cubic structure were averaging spatially or dynamically over tetragonal domains. Recently, Zhang and Scott (J.\ Phys.\ Cond.Matt.\ {\bf 19}, 275201) suggested that the lower-temperature alpha phase, widely accepted to be tetragonal (P$4_12_12$), might be an artifact in a similar way. With this motivation we investigate the energy landscape in the vicinity of cristobalite phases using first-principles calculations. We use the ABINIT implementation of density-functional theory in a plane-wave pseudopotential framework. We find that both the P$4_12_12$ alpha and I$\overline{4}$2d beta phases are local minima, thus reinforcing that the identification of the alpha phase as belonging to the P$4_12_12$ structure. We compute the frequencies of phonon modes at high-symmetry k-points in both structures and compare with experiment. We also identify a minimum-energy path connecting the alpha and beta phases through an intermediate orthorhombic phase (P$2_12_12_1$), and find a surprisingly low barrier of $\sim$5\,meV per formula unit. We note that a simple rigid-unit mode picture gives a good rough description of these energetics, and we map out the minimum-energy path in the space of rigid unit rotations in a physically insightful way. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B38.00003: First Principles Modeling of BaCeO$_{3}$: Stabilization of O Vacancies Joseph Bennett, Andrew Rappe We use first-principles density functional theory (DFT) calculations to investigate the ground state structures of both BaCeO$_{3}$ (BC) and Pd-doped BC (BCP) perovskites of general formula ABO$_{3}$. The relaxed structures match closely with recent experimental scattering studies, and also provide a local picture of how the BC perovskite lattice accommodates Pd. Both stoichiometric and oxygen-deficient materials are considered, and structures with an O vacancy adjacent to each Pd are predicted to be favored. The oxidation state of Pd in each doped structure is investigated through a structural analysis, the results of which are supported by an orbital-resolved projected density of states. The local bonding environments around Pd as well as the electron fillings of atomic orbitals on Pd and its neighbors are examined. A particular focus is the character of the highest occupied molecular orbital (HOMO) and the few lowest unoccupied molecular orbitals (LUMO). These electronic states are compared with expectations based on crystal field splittings in the computed atomic geometries. The vacancy stabilization by Pd in BCP is explained through redox chemistry and lattice strain relief. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B38.00004: Dynamical structure factor of LiF for all wave vector transfers: New results and insights Q. Kou, M.C. Troparevsky, A.G. Eguiluz(1), B.C. Larson, J.Z. Tischler, P. Zschack We report a theoretical-experimental investigation of the dynamical structure factor of LiF. The calculations are done within the TDLDA of time-dependent density functional theory; the measurements correspond to non-resonant inelastic x-rays scattering (NIXS) cross sections, obtained in absolute units. The TDLDA spectra contain one adjustable parameter: a ``scissors-operator'' shift of the conduction bands. This parameter is determined in view of the NIXS line shape for $q$= 6{\AA}$^{-1}$ ($q //$(111)). The TDLDA spectra display a non-trivial semi-quantitative agreement with the NIXS data, for all wave vectors (directed along the three high-symmetry directions); indeed, the line shape changes drastically from the coherent-response small-$q$ regime, to the incoherent-response large-$q$ regime. The picture of the excitations which emerges offers an alternative view relative to a seminal investigation involving an approximate solution of the Bethe-Salpeter equation; W. A. Caliebe et al. Phys. Rev. Lett. 84, 3907 (2000). [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B38.00005: Simulation of Li ion diffusion near electrolyte-metal interface -- Li$_3$PO$_4$ and Li Yaojun Du, Xiao Xu, N.A.W. Holzwarth Motivated by recent technological interest in Li$_3$PO$_4$-based electrolytes 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 rechargeable solid-state batteries and other technologies, we have used first-principles modeling techniques to study Li ion diffusion near idealized interfaces between Li$_3$PO$_4$ and Li metal. Using the nudged elastic band method, migration energy barriers for Li ion diffusion across interfaces in different crystallographic directions are calculated for both vacancy and interstitialcy mechanisms. Preliminary results find interface migration barriers as low as 0.2 eV and 0.26 eV for the vacancy and interstitialcy mechanisms, respectively. This suggests that interface diffusion barriers are likely to be comparable or lower than the corresponding migration barriers within crystalline Li$_3$PO$_4$.\footnote{Y. A. Du and N. A. W. Holzwarth, {\em{Phys. Rev. B}}, {\bf{76}}, 174302 (2007).} [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B38.00006: QNS study on superprotonic conductor Yutaka Ikedo, Hiroshi Nozaki, Masahiko Harada, Jun Sugiyama, Taku Sato, Yasumitsu Matsuo, Yiming Qiu, John Copley Cesium hydrogen selenate, CsHSeO$_4$, and related materials $M$H$X$O$_4$, where $M$ = Cs, Rb, K and $X$ = S, Se, are considered to be a promising candidate as electrolyte materials for fuel cells. In order to clarify the mechanism of their high proton conductivity, quasielastic neutron scattering (QNS) measurements were carried out using single crystal samples of CsHSeO$_4$ on disk chopper spectrometer (DCS) in NIST Center for Neutron Research at temperatures mainly above T$_C$ (= 401 K), at which CsHSeO$_4$ undergoes a structural phase transition from a low-T orthorhombic phase (Phase II) to a high-T tetragonal phase (Phase I). High proton conductivity, i.e., super-protonic ionic conductivity is observed only in Phase I. The analysis of the QNS spectrum suggests an anisotropic proton diffusion in Phase I, in spite of the isotropic behavior reported by AC conductivity measurements. The present QNS result thus provides crucial information on the proton diffusion pass in these materials. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B38.00007: Proton Transport in Mixed Rare-Earth/Alkaline Earth Metaphosphate Glasses Gabriel Harley, Lutgard C. De Jonghe The transport properties of [La$_{(1-x)}$M$_{x}$]-P$_{3}$O$_{9}$ metaphosphate glasses, where M$_{x}$ is Ba, Sr, Ca, and 0$\le $x$\le $0.8 were investigated in the 300 -- 500\r{ }C range. Protons are found to be incorporated as charge compensation for the substituting divalent cation. A model for proton conduction in phosphate glasses is presented where aliovalent cations within the phosphate network act as trapping sites for protons. Protons are transported via trapping center to trapping center along phosphate tetrahedra. The diffusion of protons is found to be between $\sim $10$^{-8}$ cm$^{2}$/s and $\sim $10$^{-6}$ cm$^{2}$/s in the 300-500\r{ }C range. The conductivity increases two orders of magnitude from the unsubstituted to the 60{\%} substituted glass reaching a maximum conductivity of $\sim $10$^{-6}$ S/cm at 450\r{ }C. The average transport distance between proton centers predicted by the electrical analysis is on the order of tens of nanometers, which is the same magnitude of proton-proton distance calculated from the structural data. The activation energy is found to be independent of concentration though dependant on modifying substitutional cation, and increases from 0.92 eV for Ba to 1.02 for Sr. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B38.00008: Evidence of structure dependence of hyperpolarizability in octupolar molecules Claudia Cardoso, Paulo Abreu, Fernando Nogueira It was recently reported that second-order hyperpolarizability increases significantly upon introduction of positive charges at the pyridyl end groups in trispyridyl octopolar chromophores, when compared with the neutral species. We performed ab-initio and semi-empirical calculations for the geometries, electronic localization function and first hyperpolarizabilities of a series of 6 trispyridyl molecules in neutral and protonated forms. Ab-initio calculations correctly reproduce the large hyperpolarizability values of the protonated octopolar molecules. Semi-empirical calculations predict somehow smaller hyperpolarizability values but reproduce the trend of the experimental and ab-initio values. Linear response TDDFT calculations are in good agreement with the experimental absorption spectra, reproducing the red-shift of the peaks with protonation. A correlation between the molecular structures and the first hyperpolarizability was established. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B38.00009: UV Spectra of rutile and anatase phases of TiO$_2$ using ABINIT2NBSE H.M. Lawler, J.J. Rehr, S. Dalosto, Z.H. Levine, E.L. Shirley We have developed an interface between the electronic structure code ABINIT and the optical spectroscopy code NBSE, developed at NIST. NBSE calculates dielectric spectra at various levels of theory, including iterative solution of the Bethe-Salpeter equation (BSE) for electron-hole states. From a single input file the interface executes a complete calculation, from the self-consistent ground-state to the dielectric spectrum. The interace can also treat momentum-transfer dependence of the dielectric function. As an application, we present calculations of the birefringent spectra of rutile and anatase TiO$_2$. Not surprisingly, we find that excitonic interactions dominate low-energy features of the spectra. We also address the role of planar-bonding, oxygen $\pi$-orbitals near the Fermi level, and the unfilled titanium d-band in the birefingent properties of these systems. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B38.00010: Zeeman Effect in TiO$_{2}$:Cr$^{3+}$ Michael Crawford, Xing Wei, Stan Tozer We will describe the results of Zeeman effect measurements for single crystals of rutile TiO$_{2}$ doped with Cr$^{3+}$. These measurements, performed at the National High Magnetic Field Laboratory in magnetic fields with strengths up to 45 T, utilized the near-infrared luminescence of Cr$^{3+}$ at a temperature of T = 1.4 K. The Cr$^{3+}$ luminescence spectra show the evolution with field strength of the splitting of the Cr$^{3+}$ zero-phonon line at 12,684 cm$^{-1}$ in magnetic fields applied parallel or perpendicular to the crystallographic $c$-axis. In the former case the zero-phonon line splits into four Zeeman components, while for the latter case three components appear. These results will be discussed and compared to earlier measurements made in weaker magnetic fields. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B38.00011: Raman electronic paramagnetic resonance (Raman-EPR) of Cr$^{3+}$ in ruby X. Lu, S. Venugopalan, Hyunjung Kim, M. Grimsditch, S. Rodriguez, A.K. Ramdas We have observed the Raman-EPR of the Zeeman split $^{4}$A$_{2}$ ground state of the Cr$^{3+}$ ion in Al$_{2}$O$_{3}$:Cr, i.e., ruby, exploiting the resonance conditions associated with the R$_{1}$ line. Employing a tunable dye laser with a photon energy E$_{L}$ in the vicinity of the Zeeman components of the R$_{1}$ luminescence, we observe the Stokes and anti-Stokes Raman transitions with shifts corresponding to the intra $^{4}$A$_{2}$ ground state levels split by the external magnetic field (\textbf{B}). The proximity of the incident and the scattered radiation to the Zeeman components of R$_{1}$ leads to selective dramatic resonance enhancements of the intensities of EPR transitions brought about as a function of \textbf{B} and E$_{L. }$The microscopic mechanism for the resonance enhancement involves the `in resonance' and `out resonance' conditions fulfilled by the virtual transitions from the sublevels of $^{4}$A$_{2}$ ground state to the sublevels of $^{2}$E by the incident and the scattered radiation in a two step process. Raman-EPR of the Zeeman sublevels of $^{2}$E excited state of R$_{1}$ is also observed. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B38.00012: Trivalent chromium probes in mixed dicyanoargentate-dicyanoaurate single crystals C.L. Larochelle, J.K. Krebs Single crystals of the form R[M(CN)$_2$]$_3$ (R=trivalent ion, such as a rare earth; M=Ag, Au, or both) have a layered structure consisting of alternating layers of M(CN)$_2^-$ ions and R$^{3+}$ ions. Recent work on this type of crystal has focused on energy transfer from the metal dicyanide donor to the rare earth acceptors, specifically Tb$^{3+}$, Eu$^{3+}$, Sm$^{3+}$, and Ce$^{3+}$. Crystals of this type are particularly interesting because the luminescence energies are tunable, changing with changes in the temperature as well as Ag/Au ratio. Doping these single crystals with chromium can provide an opportunity to study the crystal field strength at the ion site because the energy levels in chromium are very sensitive to the ion's environment. We present steady-state excitation and emission results, along with lifetime measurements for a series of crystals of the type Cr$_{0.1}$La$_{0.9}$[Ag$_{x}$Au$_{1-x}$(CN)$_2$]$_3$, with $x$=1, 0.5, 0.75, 0.9. These measurements indicate that the chromium emission is of an unusually low energy in these crystals. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B38.00013: Ce/Cr:YAG Based Yellow/Red Phosphors Jinke Tang, Wendong Wang, Sheng Teng Hsu, Brian Sullivan Ce:YAG (yttrium aluminum garnet) is the current industrial material of choice for phosphor for white LEDs, but suffers from poor color rendering index. We have investigated Cr and Ce co-doped YAG. Our investigation suggests Cr addition can drastically enrich the emission spectrum of Ce:YAG in the red region, which is realized by a non-radiative energy transfer from the Ce3+ 2D3/2 level to Cr3+ 4T level. The emission spectrum excited at 467 nm blue wavelength shows that the addition of Cr induces strong peaks in the red region. Comparison with the emission spectrum of the Cr-only sample reveals that the intensity of red Cr emission is much higher in Ce and Cr co-doped samples, which suggests energy transfer from Ce to Cr. Excitation spectra collected at the Cr red emission show a large excitation peak at 458 nm and suggest energy transfer from the 2D3/2 level of Ce3+ to the 4T level of Cr3+, which is responsible for the enhanced red emission from Cr. At the same time, radiative transfer from Ce3+(2D3/2) to Cr3+(4T) through the absorption of the yellow emission by Cr seems limited. The orders of magnitude increase in the Cr red emission in Ce/Cr:YAG compared to Cr:YAG suggests that the former is an efficient red and yellow phosphor. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B38.00014: Spectral Intensities of Transitions Between Stark Levels of Er$^{3+}$(4f$^{11})$ in Single Crystal, Ceramic, and Nanocrystalline Y$_{2}$O$_{3}$ Kelly Nash, John Gruber, Dhiraj Sardar, Uygun Valiev, Nikolai Ter-Gabrielyan, Larry Merkle, Arockiasamy Michael Similarities and differences among the optical properties of Er$^{3+}$:Y$_{2}$O$_{3}$ in single crystal, polycrystalline (ceramic), and nanocrystalline forms are discussed based on spectra obtained between 400 nm and 1700 nm and temperatures between 8 K and 300K. The observed crystal-field splitting and the measured intensities of transitions between the $^{2S+1}$L$_{J}$ manifolds of Er$^{3+}$(4f$^{11})$ in both the C$_{2}$ and C$_{3i}$ sites are analyzed in terms of models that invoke the mixing of states of opposite parity through the odd terms in the crystal-field Hamiltonian. The inversion symmetry of C$_{3i}$ sites limits electronic transitions to magnetic dipole transitions between the $^{4}$I$_{13/2}$ and $^{4}$I$_{15/2}$ manifolds. For Er$^{3+}$ ions in C$_{2}$ sites, the forced electric-dipole transitions along with some magnetic dipole contribution in certain cases, are allowed between the J+1/2 Stark levels within all manifolds. Within the instrumental resolution, there are some important differences between intensities of transitions depending on particle size of the Er$^{3+:}$Y$_{2}$O$_{3}$. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B38.00015: Judd-Ofelt analysis and crystal-field modeling of Er$^{3+}$ transitions in YAlO$_{3}$ Sree R. Chandrasekharan Optical absorption and emission intensities are investigated for trivalent Er$^{3+}$ ions in YAlO$_{3}$ crystal. The Judd-Ofelt model is applied to the room temperature absorption intensities of Er$^{3+}$(4f$^{11})$ transitions in YAlO$_{3}$ to obtain the intensity parameters which are then used to calculate the spontaneous emission probabilities, branching ratios, radiative decay rates, and radiative lifetimes of the Er$^{3+}$ transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds in YAlO$_{3}$. The room-temperature fluorescence lifetimes and the emission cross sections of selected intermanifold transitions are determined. From the calculated radiative lifetimes and the measured fluorescence lifetimes, the quantum efficiency of the sample has been found. The 8K absorption spectrum has been examined as well. Selected manifolds have been analyzed in terms of crystal field splitting using current models and minimization methods to establish the parameters of Er$^{3+}$ in C$_{s}$ symmetry sites. The optical and spectroscopic characteristics of Er$^{3+}$:YAlO$_{3 }$show that this material has a potential for both 1.5$\mu $m and 544.96 nm stimulated emissions. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700