T. Vogt - Academia.edu (original) (raw)

Papers by T. Vogt

Physical Review B, 1996

Magnetic excitations in a Nd 2 BaNiO 5 powder sample were investigated by inelastic neutron scatt... more Magnetic excitations in a Nd 2 BaNiO 5 powder sample were investigated by inelastic neutron scattering. Well-defined one-dimensional ͑1D͒ gap modes were observed above and below the Néel temperature T N ϭ24 K. In both temperature regimes the measured dynamic structure factor was found to be consistent with that calculated for transverse Haldane gap modes in a quantum-disordered Sϭ1 linear-chain antiferromagnet. The energy gap does not show any appreciable T dependence at TϾT N. However, in the ordered phase the spin gap increases. The observed behavior is totally different from that found in other Haldane gap systems undergoing 3D antiferromagnetic ordering, such as CsNiCl 3. ͓S0163-1829͑96͒06134-6͔

Nanostructure Science and Technology, 2012

We outline a new systematic approach to extracting high quality information from HAADF-STEM image... more We outline a new systematic approach to extracting high quality information from HAADF-STEM images which will be beneficial to the characterization of beam sensitive materials. The idea is to treat several, possibly many low electron dose images with specially adapted digital image processing concepts at a minimum allowable spatial resolution. Our goal is to keep the overall cumulative electron dose as low as possible while still staying close to an acceptable level of physical resolution. We shall present the main conceptual imaging concepts and restoration methods that we believe are suitable for carrying out such a program and, in particular, allow one to correct special acquisition artifacts which result in blurring, aliasing, rastering distortions and noise.

Physical Review B, 2003

We have prepared powder samples of Na x CoO 2 x yH 2 O using a new synthesis route. Superconducti... more We have prepared powder samples of Na x CoO 2 x yH 2 O using a new synthesis route. Superconductivity was observed in Na 0.3 CoO 2 x 1.3H 2 O between 4 and 5K as indicated by the magnetic susceptibility. The bulk compressibilities of Na 0.3 CoO 2 x 1.3H 2 O, Na 0.3 CoO 2 x 0.6H 2 O and Na 0.3 CoO 2 were determined using a diamond anvil cell and synchrotron powder diffraction. Chemical changes occurring under pressure when using different pressure transmitting media are discussed and further transport measurements are advocated.

Proceedings of the National Academy of Sciences, 2010

In this work, we structurally characterize defects, grain boundaries, and intergrowth phases obse... more In this work, we structurally characterize defects, grain boundaries, and intergrowth phases observed in various Mo-V-O materials using aberration-corrected high-angle annular dark-field (HAADF) imaging within a scanning transmission electron microscope (STEM). Atomic-level imaging of these preparations clearly shows domains of the orthorhombic M1-type phase intergrown with the trigonal phase. Idealized models based on HAADF imaging indicate that atomic-scale registry at the domain boundaries can be seamless with several possible trigonal and M1-type unit cell orientation relationships. The alignment of two trigonal domains separated by an M1-type domain or vice versa can be predicted by identifying the number of rows/columns of parallel symmetry operators. Intergrowths of the M1 catalyst with the M2 phase or with the Mo 5 O 14 -type phase have not been observed. The resolution enhancements provided by aberration-correction have provided new insights to the understanding of phase eq...

Journal of Solid State Chemistry, 2005

Understanding the stability of volatile species and their compounds under various surface and sub... more Understanding the stability of volatile species and their compounds under various surface and subsurface conditions is of great importance in gaining insights into the formation and evolution of planetary and satellite bodies. We report the experimental results of the temperature-and pressure-dependent structural transformation of methane hydrates in salt environments using in situ synchrotron X-ray powder diffraction, solid-state nuclear magnetic resonance, and Raman spectroscopy. We find that under pressurized and concentrated brine solutions methane hydrate forms a mixture of type I clathrate hydrate, ice, and hydrated salts. Under a low-pressure condition, however, the methane hydrates are decomposed through a rapid sublimation of water molecules from the surface of hydrate crystals, while NaCl • 2H 2 O undergoes a phase transition into a crystal growth of NaCl via the migration of salt ions. In ambient pressure conditions, the methane hydrate is fully decomposed in brine solutions at temperatures above 252 K, the eutectic point of NaCl • 2H 2 O. 1. Introduction Clathrate hydrates, which possess similar chemical properties to ice, can be stabilized using the van der Waals interaction between the small guest molecules (CH 4 , CO 2 , H 2 , N 2 , etc.) and the host water framework constructed by the hydrogen bonds in the water molecules [Berecz and Bella-Achs, 1983; Sloan, 2003; Sloan and Koh, 2008]. Several types of guest molecules are known to form the three common structures of clathrate hydrates, which are classified into structure I (sI), structure II (sII), and structure H (sH) hydrates [Sloan, 2003; Sloan and Koh, 2008]. In the icy matrix of clathrate hydrates, vast amounts of gas storage and selective guest enclathration from mixtures are possible, which has resulted in these hydrates garnering considerable attention in the energy and environmental fields over the past several decades [

Chemistry of Materials, 1998

Sm, and Y 0.5 Sm 0.5) have been prepared, and structure determinations have been carried out at r... more Sm, and Y 0.5 Sm 0.5) have been prepared, and structure determinations have been carried out at room temperature using high-resolution synchrotron X-ray powder diffraction data. The octahedral tilting distortion increases as the average ionic radius of the Ln/A cations, 〈r A 〉, decreases. The two crystallographically distinct Mn-O-Mn bonds [Mn-O(eq)-Mn and Mn-O(ax)-Mn] are almost identical for Ln 0.5 Ca 0.5 MnO 3 compounds, with the exception of La 0.5 Ca 0.5 MnO 3. The La 0.5 Ca 0.5 MnO 3 compound and the entire Ln 0.5 Sr 0.5 MnO 3 series adopt structures where the Mn-O(eq)-Mn bond angle is consistently and significantly larger (2-6°) than the Mn-O(ax)-Mn bond angle. All of the Ln 0.5 Ca 0.5 MnO 3 compounds have Pnma symmetry, whereas across the Ln 0.5 Sr 0.5 MnO 3 series with increasing 〈r A 〉, an evolution from Pnma (tilt system ab + a-) over Imma (tilt system ab 0 a-) to I4/mcm (tilt system a 0 a 0 c-) symmetry is observed. It appears that the latter two tilt systems are stabilized with respect to the rhombohedral (R3 hc) aaatilt system, by short-range layered ordering of A-site cations. Changes in the octahedral tilt system at room temperature are linked to changes in the low-temperature magnetic structure. In particular, the simultaneous onset of charge ordering and CE-type antiferromagnetism in the Ln 0.5 Sr 0.5 MnO 3 series appears to be closely associated with the Imma structure. The average Mn-O bond distance is relatively constant across the entire series, but individual Mn-O bond distances show the presence of a cooperative Jahn-Teller effect (orbital ordering) at room temperature in Sm 0.5 Ca 0.5 MnO 3 and Sm 0.25 Y 0.25 Ca 0.5 MnO 3 .

Physical Review B, 2003

The cubic perovskite-related ceramic CaCu3Ti4O12 has a very high static dielectric constant ε0 > ... more The cubic perovskite-related ceramic CaCu3Ti4O12 has a very high static dielectric constant ε0 > ∼ 10 000 at room temperature (RT), which drops to about 100 below ≃ 100 K. Substituting Cd for Ca reduces the RT value of ε0 by over an order of magnitude. The large ε0 may be due to an internal barrier layer capacitance (IBLC) effect. Infrared optical properties show a low-frequency mode that increases dramatically in strength at low temperature, suggesting a change in the effective charges and a breakdown of the IBLC model due to a semiconductor-to-insulator transition.

Physics and Chemistry of Minerals, 2004

A polycrystalline sample of natural laumontite (Pleasant Valley, Connecticut) was studied up to 6... more A polycrystalline sample of natural laumontite (Pleasant Valley, Connecticut) was studied up to 6.8 (1) GPa at room temperature using monochromatic synchrotron X-ray powder diffraction and a diamond-anvil cell. A methanol: ethanol: water mixture was used as a penetrating pressure-transmitting fluid. A dry sample measured before adding the pressure fluid inside the diamond-anvil cell contained $12 H 2 O per formula unit, consistent with the water content of partially dehydrated laumontite. Upon increasing the pressure to 0.2 (1) GPa, fully hydrated laumontite with 18 H 2 O per unit cell formed and the unit-cell volume expanded by 2.6%. Further pressure increase up to 2.4 (1) GPa resulted in a gradual contraction of the unit-cell volume and individual cell lengths. During this process, a successive order-disorder transition of hydrogen-bonded water molecules from their symmetry sites was observed, concomittent with an inflectional behavior of the monoclinic beta angle and the channel ellipticity. Above 3 GPa, a tripling of the b axis was detected. Thereafter, up to 6.8 (1) GPa, the compression behavior was reversed for the c axis length and the monoclinic beta angle which showed a gradual increase and decrease, respectively, without any apparent volume discontinuity. We suspect that different ordering of the water molecules or Ca cations inside the channels along the b axis may be responsible for the observed supercell transition above 3 GPa.

Physical Review B, 2006

Pressure-induced structural, electronic, and thermodynamic changes in ␣-AlH 3 were investigated u... more Pressure-induced structural, electronic, and thermodynamic changes in ␣-AlH 3 were investigated using synchrotron x-ray powder diffraction and density-functional theory. No first-order structural transitions were observed up to 7 GPa. However, increasing Bragg peak asymmetry with pressure suggests a possible monoclinic distortion at moderate pressures ͑1-7 GPa͒. The pressure-volume relationship was fit to the Birch-Murnaghan equation of state to give a bulk modulus of approximately 40 GPa. The reduced cell volume at high pressure is accommodated by octahedral tilting and a decrease of the Al-H bond distance. Ab initio calculations of the free energy indicate that hydrogenation becomes favorable at H 2 pressures above 0.7 GPa at 300 K. Electronic density of states calculations reveal a slight decrease in the band gap with pressure but no evidence of an insulator-to-metal transition predicted by previous high-pressure studies. Calculated Mulliken charges and bond populations suggest a mixed ionic and covalent Al-H bond at 1 atm with an increase in covalent character with pressure.

Journal of the American Chemical Society, 2001

4 cell length (Å) a 18.226(2) 18.180(4) 18.390(6) 18.378(3) 18.233(3) 18.097(2) 17.924(2) b 18.58... more 4 cell length (Å) a 18.226(2) 18.180(4) 18.390(6) 18.378(3) 18.233(3) 18.097(2) 17.924(2) b 18.583(2) 18.531(4) 18.829(6) 18.818(3) 18.679(3) 18.518(2) 18.325(2) c 6.579(1) 6.566(2) 6.547(2) 6.545(1) 6.530(1) 6.512(1) 6.487(1) Si(1) 8a x 0 0 0 0 0 0 0 Si(2) x 0.1493(1) 0.1491(1) 0.1536(1) 0.1534(1) 0.1505(4) 0.1510(2) 0.1507(2) 16b y 0.2168(1) 0.2177(1) 0.2106(1) 0.2107(1) 0.2134(4) 0.2128(2) 0.2128(2) z 0.6180(2) 0.6177(2) 0.6172(2) 0.6172(1) 0.6173(9) 0.6175(3) 0.6166(3) Al x 0.0338(1) 0.0331(1) 0.0400(1) 0.0399(1) 0.0376(4) 0.0375(2) 0.0368(2) 16b y 0.0975(1) 0.0977(1) 0.0929(1) 0.0930(1) 0.0952(4) 0.0957(2) 0.0963(2) z 0.6095(2) 0.6097(2) 0.6099(2) 0.6099(1) 0.6085(9) 0.6098(3) 0.6103(3) O(1) x 0.0170(4) 0.0131(3) 0.0400(3) 0.0400(2) 0.0383(8) 0.0404(4) 0.0435(3) 16b y 0.0695(1) 0.0705(1) 0.0588(2) 0.0588(1) 0.0605(5) 0.0602(3) 0.0590(2) z 0.8592(2) 0.8589(2) 0.8586(2) 0.8585(1) 0.8577(9) 0.8587(4) 0.8579(3) O(2) x 0.0662(1) 0.0660(1) 0.0711(1) 0.0708(1) 0.0668(4) 0.0659(2) 0.0641(2) 16b y 0.1861(1) 0.1864(1) 0.1807(1) 0.1809(1) 0.1844(4) 0.1860(2) 0.1879(2) z 0.6080(3) 0.6078(3) 0.6079(3) 0.6079(2) 0.6090(16) 0.6077(6) 0.6072(6) O(3) x 0.0991(1) 0.0995(1) 0.0968(1) 0.0969(1) 0.0969(6) 0.0963(3) 0.0946(3) 16b y 0.0404(2) 0.0398(2) 0.0408(1) 0.0410(1) 0.0444(4) 0.0450(2) 0.0467(2) z 0.4997(9) 0.5072(7) 0.4565(6) 0.4564(3) 0.4556(19) 0.4525(8) 0.4439(7) O(4) x 0.2018(2) 0.2025(2) 0.2011(1) 0.2009(1) 0.1980(4) 0.1967(2) 0.1942(2) 16b y 0.1569(1) 0.1568(1) 0.1596(1) 0.1596(1) 0.1606(6) 0.1597(3) 0.1600(3) z 0.7219(9) 0.7143(7) 0.7650(6) 0.7652(3) 0.7634(18) 0.7686(8) 0.7772(7) O(5) x 0.1791(1) 0.1781(1) 0.1898(2) 0.1897(1) 0.1878(5) 0.1885(3) 0.1900(2) 16b y 0.2335(4) 0.2374(3) 0.2111(3) 0.2111(2) 0.2126(7) 0.2105(4) 0.2072(3) z 0.3909(2) 0.3912(2) 0.3917(2) 0.3917(1) 0.3915(9) 0.3917(4) 0.3922(3) Na x 0.220(1) 0.221(1) 0.224(1) 0.224(1) 0.217(1) 0.228(2) 0.225(1) 16b y 0.035(1) 0.036(1) 0.028(1) 0.029(1) 0.029(1) 0.024(1) 0.025(1) z 0.623(2) 0.638(4) 0.629(5) 0.599(4) 0.557(4) 0.615(3) 0.623(3) OW1 x 0.059(1) 0.054(2) 0.055(2) 0.055(2) 0.065(2) 0.061(1) 0.060(1) 16b y 0.190(1) 0.184(2) 0.181(2) 0.180(1) 0.178(2) 0.182(1) 0.186(1) z 0.110(4) 0.139(8) 0.12(1) 0.107(8) 0.113(9) 0.113(7) 0.117(6) OW2 x 0.184(2) 0.180(2) 0.190(1) 0.180(1) 0.181(1) 16b y 0.071(2) 0.066(1) 0.067(2) 0.070(1) 0.081(1) z 0.110(6) 0.086(5) 0.054(6) 0.110(4) 0.088(4) a Esd's are in parentheses. Fixed isotropic displacement parameters, U iso (Å 2), were used for all models (0.017 and 0.022 for framework and extra-framework elements, respectively). All sites are fully occupied. b Generated from starting framework models.

Journal of Solid State Chemistry, 1992

This investigation presents the preparation of CeN0.222O0.667F1.333 by a solid-state reaction fro... more This investigation presents the preparation of CeN0.222O0.667F1.333 by a solid-state reaction from a mixture of CeN:CeF3:CeO2 = 1:2:1.5 and its structural investigation. The samples were annealed at 900°C in platinum tubes for different times. The basic structure found by powder neutron diffraction is anion-excess fluorite-related. The unit cell is an orthorhombic distortion of the cubic fluorite cell and has the space group Abm2. The lattice constants are a = 577.71(2) pm, b = 572.76(5) pm, and c = 573.32(6) pm. The structure refined by Rietveld analysis shows that [1:0:2]- defect clusters are present. In samples prepared by longer annealing times an ordering of these clusters to larger aggregates, i.e., toward the vernier phases, was observed. This was deduced from full profile analysis without refining a structural model by comparing the instrumental resolution curves of several models.

Journal of Solid State Chemistry, 1989

Crystals of K1.35Tis016 have been investigated by single-crystal X-ray analysis, X-ray powder pat... more Crystals of K1.35Tis016 have been investigated by single-crystal X-ray analysis, X-ray powder patterns, electron diffraction, and high-resolution electron microscopy. K1.35Ti8016 is obtained in the form of dark blue tetragonal crystals of the space group 14/m having the hollandite structure with a = 1018.8(2) pm and c = 2%.61(7) pm. The refinement converges to a reliability factor of 0.025. The occupation of the T(2,2) tunnel sites in potassium is not statistical. Incommensurate one-dimensional superstructures were found by powder diffraction patterns as well as by electron diffraction. The multiplicity calculated from powder X-ray diffraction is m = 5.79 and that found by electron diffraction is m = 8.1; this is explained in terms of beam damage and loss of potassium within the T(2,2) sites. The final step in the decomposition is the formation of rutile from K1.)STi8016 when all the potassium atoms are lost in the tunnels. 6 1989 Academic Press. Inc.

Journal of Solid State Chemistry, 2002

Double-cell perovskites RBaFe 2 O 5þw (R = Nd and Sm) are synthesized in the À0:03owo0:47 portion... more Double-cell perovskites RBaFe 2 O 5þw (R = Nd and Sm) are synthesized in the À0:03owo0:47 portion of the nonstoichiometry range. The ideal composition with w ¼ 0 has di-and trivalent iron in equal proportions and exhibits a charge-ordering Verwey transition upon cooling, preceded by a weaker premonitory transition. Both transitions are detectable by differential scanning calorimetry. The changes in entropy, volume, orthorhombic distortion and electrical conductivity at the main transition are larger for the variant with the smaller Sm atom than for R=Nd. The discontinuity of the SmBaFe 2 O 5þw transition also persists into much higher levels of the oxygen nonstoichiometry w than for the R=Nd variant. Whereas 3D-charge-ordered SmBaFe 2 O 5:00 is isostructural with TbBaFe 2 O 5:00 adopting space group Pmma; NdBaFe 2 O 5:00 undergoes a more profound structural change upon entering the charge-ordered state, the structure of which is refined in the P2 1 ma symmetry from high-resolution X-ray synchrotron diffraction data.

Journal of Physics: Condensed Matter, 2001

... Res. 8 667 CrossRef [22] Howard CJ and Hunter BA 1998 A Computer Program for Rietveld Analysi... more ... Res. 8 667 CrossRef [22] Howard CJ and Hunter BA 1998 A Computer Program for Rietveld Analysis of X-ray and Neutron Powder Diffraction Patterns (NSW, Australia: Lucas Heights Research Laboratories) [23] Lerch M, Boysen H and Hansen T 2001 J. Phys. Chem. ...