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Sökning: LAR1:uu > Chalmers tekniska högskola > Eriksson Sten G > Uppsala universitet

  • Resultat 1-10 av 13
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  • Ahmed, Istaq, 1972-, et al. (författare)
  • Synthesis and structural characterization of perovskite type proton conducting BaZr1−xInxO3−δ (0.0 ≤ x ≤ 0.75)
  • 2006
  • Ingår i: Solid State Ionics. - 0167-2738. ; 177:17-18, s. 1395-1403
  • Tidskriftsartikel (refereegranskat)abstract
    • Solid state sintering has been used to prepare the cubic perovskite structured compounds BaZr1-xInxO3-delta (0.0 <= x <= 0.75). Analysis of X-ray powder diffraction (XRPD) data reveals that the unit cell parameter, a, increases linearly with an increased Indium concentration. XRPD data was also used to demonstrate the completion of sample hydration, which was reached when the materials showed a set of single-phase Bragg-peaks. Dynamic thermogravimetric analysis (TGA) data showed that approx. 89% of the total number of available oxygen vacancies can be filled in BaZr1-xInxO3-delta for x=0.50, and that the maximum water uptake occurs below 300 degrees C. Rietveld analysis of the room temperature neutron powder diffraction (NPD) data confirmed the average cubic symmetry (space group Pm-3m), and an expansion of the unit cell parameter after the hydration reaction. The strong O-H stretch band, 2500-3500 cm(-1), in the infrared absorbance spectrum clearly manifests the presence of protons in the hydrated material. Proton conductivity of hydrated BaZr1-xInxO3-delta, x=0.75 was investigated during heating and cooling cycles under dry argon atmosphere. The total conductivity during the heating cycle was nearly two orders of magnitude greater than that of cooling cycle at 300 degrees C, whilst these values were similar at higher temperatures i.e. T > 600 degrees C.
  • Andreasson, Jakob, 1975-, et al. (författare)
  • Electron-lattice interactions in the perovskite LaFe0.5Cr0.5O3 characterized by optical spectroscopy and LDA+U calculations
  • 2009
  • Ingår i: Physical Review B. - 1098-0121. ; 80:7
  • Tidskriftsartikel (refereegranskat)abstract
    • We use resonance Raman scattering (incident photon energies between 1.8 and 4.13 eV), LDA+U calculations, spectroscopic ellipsometry, and oblique IR reflectivity to characterize the strong electron-phonon interactions in the disordered perovskite LaFe0.5Cr0.5O3. When the photon energy coincides with a Cr to Fe Mott-Hubbard transfer gap around 2.4 eV the electron-phonon interaction is manifested by a Franck-Condon effect with exceptional first-and higher order scattering of a local oxygen breathing mode. At higher incident energies we observe a superposition of Franck-Condon scattering and Frohlich interaction induced infrared active longitudinal optical two-phonon scattering activated mainly by O to Fe charge transfer. Our results establish LaFe0.5Cr0.5O3 as a model compound for research on electron-phonon interactions in strongly correlated complex systems and show that Franck-Condon scattering in complex solids is not limited to Jahn-Teller active compounds.
  • Andreasson, Jakob, 1975-, et al. (författare)
  • Electron-phonon interactions in perovskites containing Fe and Cr studied by Raman scattering using oxygen-isotope and cation substitution
  • 2008
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 78:23
  • Tidskriftsartikel (refereegranskat)abstract
    • We use temperature-dependent inelastic light scattering to study the origin of the strong multiphonon scattering of a local oxygen breathing mode present in the mixed B-site orthorhombic (space group Pnma) perovskite LaFe0.5Cr0.5O3 but absent in isostructural LaFeO3 and LaCrO3. It is seen that the multiphonon scattering is critically sensitive to the presence of both Fe and Cr ions on the B site. These results support our interpretation that the multiphonon scattering is activated by local electron-phonon interactions according to the Franck-Condon picture following an Fe-Cr charge transfer. Further, O-18 substitution is performed on the x=0, 0.04, and 0.5 compounds and clearly shows that all modes appearing above the first-order phonon-scattering region in these compounds originate from higher-order oxygen stretching vibrations. In particular this is the case for the strong second-order scattering dominating the scattering response in LaFeO3. Accordingly we propose that these modes are generated by infrared-active longitudinal optical (IR LO) two-phonon and combination scattering activated by Frohlich interaction. For x=0.02 and 0.04 the characteristic IR LO two-phonon and Franck-Condon multiphonon-scattering profiles mix. We also study the influence of isovalent cation substitution and Sr doping in AFe(0.5)Cr(0.5)O(3) (A=La, Nd, and Gd) and La1-ySryFe0.5Cr0.5O3-delta (y=0, 0.16, and 0.5) on the strong electron-phonon coupling present in LaFe0.5Cr0.5O3. The Franck-Condon effect in LaFe0.5Cr0.5O3, is not significantly affected by isovalent A-site substitution, despite the increasing orthorhombic distortion associated with decreasing A-site ionic radii. On the contrary, aliovalent Sr doping causes a rapid decrease in the Franck-Condon scattering. This shows that the strong electron-phonon coupling in these compounds is highly sensitive to local lattice and electronic decoherence but insensitive to global lattice distortions. Finally, a preliminary assignment of the A(g) and B-2g phonon modes in AFe(0.5)Cr(0.5)O(3) (A=La, Nd, and Gd) is made based on the present observations and published results for LaCrO3 and AMnO(3). The modes associated with oxygen octahedral tilt and bending vibrations are heavily influenced by the magnitude of the orthorhombic distortion.
  • Andreasson, Jakob, et al. (författare)
  • Franck-Condon higher order lattice excitations in the LaFe(1-x)Cr(x)O3 (x=0, 0.1, 0.5, 0.9, 1.0) perovskites due to Fe-Cr charge transfer effects
  • 2007
  • Ingår i: Physical Review B. ; 75
  • Tidskriftsartikel (refereegranskat)abstract
    • First and higher order lattice excitations in the B-site disordered perovskites LaFe1-xCrxO3 (x=0, 0.1, 0.5, 0.9, and 1) and La0.835Sr0.165Fe0.5Cr0.5O3-delta are investigated using temperature dependent and polarized inelastic light scattering [lambda=515 nm (2.41 eV) and 676 nm (1.83 eV)] on oriented crystallites. A peak at approximately 2.4 eV in the imaginary part of the dielectric function of LaFe0.5Cr0.5O3 is assigned to a charge transfer from Fe3+ (d(5)) to Cr3+ (d(3)) ions, coupled with the appearance of an intense A(g)-like mode at approximately 700 cm(-1) in the Raman data. This excitation is identified as a symmetric oxygen breathing mode activated by the Fe-Cr charge transfer through an orbital coupling mechanism. Higher order scattering (up to seventh order) of the intrinsic Raman active symmetric breathing mode is also explained by an orbital-mediated electron-phonon coupling, similar to the Franck-Condon effect observed in the Jahn-Teller active-perovskite-structured manganite LaMaO(3). These results show that the Franck-Condon mechanism is a more common mechanism for resonant higher order scattering in solids than previously believed and propose the LaFe1-xCrxO3 system as a model system for electron-phonon coupling and higher order Raman scattering in solids.
  • Azad, A.K., et al. (författare)
  • Spin-glass transition in a La-doped Sr2MnWO6 double perovskite
  • 2008
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 77:6, s. 064418
  • Tidskriftsartikel (refereegranskat)abstract
    • Polycrystalline Sr2-xLaxMnWO6 (x=0.25 and 0.5) materials have been prepared by a traditional solid state sintering method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature dependent NPD data shows that the compounds crystallize in monoclinic symmetry (space group P2(1)/n). The unit cell volume decreases with increasing La3+ concentration at the A site. The crystal structure contains alternating MnO6 and WO6 octahedra, considerably tilted due to the relative small size of the cations that occupy the A sublattice of the perovskite. ac and dc magnetization measurements show a destruction of the low temperature antiferromagnetic phase by electron doping through substitution of Sr2+ for La3+. The electron doping creates a mixed tungsten valence (W6+/W5+) resulting in a low temperature spin-glass state.
  • Bielecki, Johan, 1982-, et al. (författare)
  • Structural and magnetic properties of isovalently substituted multiferroic BiFeO3: Insights from Raman spectroscopy
  • 2012
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 86:184422
  • Tidskriftsartikel (refereegranskat)abstract
    • Raman spectra, supplemented by powder x-ray diffraction and magnetization data of isovalently A- and B-site substituted BiFeO3 in the Bi1−xLaxFeO3 (0≤x≤1), Bi1−xTbxFeO3 (0≤x≤0.2), and Bi0.9Sm0.1Fe1−xMnxO3 (0≤x≤0.3) series, are presented. A good agreement between the structural transitions observed by x-ray diffraction and the vibrational modes observed in the Raman spectra is found over the whole substitutional ranges, and in particular we find spectroscopic signatures of a PbZrO3-type structure for Bi0.8La0.2FeO3. Mode assignments in the substituted materials are made based on Raman spectra of the end-members BiFeO3 and LaFeO3. Moreover, by comparing spectra from all samples with R3c structure, the phonon assignment in BiFeO3 is revisited. A close connection between the degree of octahedral tilt and the Raman shift of the A1 oxygen a−a−a− tilt mode is established. An explanation for the strong second-order scattering observed in Bi1−xLaxFeO3 and Bi1−xTbxFeO3 is suggested, including the assignment of the previously mysterious BiFeO3 mode at 620 cm−1. Finally, the magnetization data indicates a transition from a cycloidal modulated state towards a canted antiferromagnet with increasing A-site substitution, while Bi0.9Sm0.1Fe1−xMnxO3 with x=0 and 0.15 exhibit an anomalous closing of the hysteresis loop at low temperatures. For low A-site substitution levels (x≤0.1) the decreasing Raman intensity of the Fe derived modes correlates with the partial destruction of the spin cycloid as the substitution level increases.
  • Biendicho, Jordi Jacas, et al. (författare)
  • In situ investigation of commercial Ni(OH)(2) and LaNi5-based electrodes by neutron powder diffraction
  • 2015
  • Ingår i: Journal of Materials Research. - 0884-2914. ; 30:3, s. 407-416
  • Tidskriftsartikel (refereegranskat)abstract
    • Electrochemical reactions at both positive and negative electrodes in a nickel metal hydride (Ni-MH) battery during charge have been investigated by in situ neutron powder diffraction. Commercially available beta-Ni(OH)(2) and LaNi5-based powders were used in this experiment as positive and negative electrodes, respectively. Exchange of hydrogen by deuterium for the beta-Ni(OH)(2) electrode was achieved by ex situ cycling of the cell prior to in situ measurements. Neutron diffraction data collected in situ show that the largest amount of deuterium contained at the positive electrode is de-intercalated from the electrode with no phase transformation involved up to similar to 100 mA h/g and, in addition, the 110 peak width for the positive electrode increases on charge. The negative electrode of composition MmNi(3.6)Al(0.4)Mn(0.3)Co(0.7), where Mm = Mischmetal, exhibits a phase transformation to an intermediate hydride gamma phase first and then to the beta phase on charge. Unit cell dimensions and phase fractions have been investigated by Rietveld refinement of the crystal structure.
  • Biendicho, Jordi Jacas, et al. (författare)
  • New in-situ neutron diffraction cell for electrode materials
  • 2014
  • Ingår i: JOURNAL OF POWER SOURCES. - 0378-7753. ; 248, s. 900-904
  • Tidskriftsartikel (refereegranskat)abstract
    • A novel neutron diffraction cell has been constructed to allow in-situ studies of the structural changes in materials of relevance to battery applications during charge/discharge cycling. The new design is based on the coin cell geometry, but has larger dimensions compared to typical commercial batteries in order to maximize the amount of electrode material and thus, collect diffraction data of good statistical quality within the shortest possible time. An important aspect of the design is its modular nature, allowing flexibility in both the materials studied and the battery configuration. This paper reports electrochemical tests using a Nickel-metal-hydride battery (Ni-MH), which show that the cell is able to deliver 90% of its theoretical capacity when using deuterated components. Neutron diffraction studies performed on the Polaris diffractometer using nickel metal and a hydrogen-absorbing alloy (MH) clearly show observable changes in the neutron diffraction patterns as a function of the discharge state. Due to the high quality of the diffraction patterns collected in-situ (i.e. good peak-to-background ratio), phase analysis and peak indexing can be performed successfully using data collected in around 30 min. In addition to this, structural parameters for the beta-phase (charged) MH electrode obtained by Rietveld refinement are presented.
  • Eriksson, Annika K., 1975-, et al. (författare)
  • Influence of iron addition on the oxygen-deficient Sr0.85Bi0.15Co1-xFexO3-delta (0.0 <= x <= 1.0) perovskites
  • 2008
  • Ingår i: Journal of Solid State Chemistry. - 0022-4596. ; 181:8, s. 2031-2040
  • Tidskriftsartikel (refereegranskat)abstract
    • A series of oxygen-deficient Sr0.85Bi0.15Co1-xFexO3-delta (0.0 <= x <= 1.0) perovskite phases were prepared using solid-state reaction. Results of neutron powder diffraction analyses show that the introduction of Fe onto the B-site severely effects the long range coherence of the oxygen vacancy ordered, 14/mmm supercell, observed for the x = 0.0 sample. For x = 0.1 a smaller, a = b approximate to a(p), c approximate to 2a(p), P4/mmm supercell gives the best agreement to the diffraction data, whilst phases in the range 0.2 <= x <= 0.6 adopt disordered cubic perovskite structures. Pseudo-cubic, a = b approximate to a(p), c approximate to a(p), structures are found for x >= 0.8. Evidence of weak superstructures, reflecting local oxygen ordering, is also obtained from electron diffraction. For all oxygen-annealed phases the average structure reverts to cubic Pm (3) over barm. The as-prepared samples show G-type antiferromagnetic order at room temperature. The oxygen annealed x = 0.10, 0.25 and 1.0 samples display low-temperature spin-glass transitions.
  • Ivanov, SA, et al. (författare)
  • Ba3Fe2WO9-delta : Effect of oxygen non-stoichiometry on structural and magnetic properties
  • 2006
  • Ingår i: Journal of Solid State Chemistry. - 0022-4596. ; 179:8, s. 2645-2655
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic and structural properties of oxygen-deficient perovskites with composition Ba3Fe2WO9-6 (BFWO) have been systematically studied for two different oxygen contents corresponding to delta = 0.00 and 0.55 in the chemical formula in order, to determine and correlate their chemical composition, structural and magnetic properties. The evolution of nuclear and magnetic structures with temperature has been investigated by neutron powder diffraction. It was shown that at room temperature the stoichiometric compound (delta = 0.00) adopts a hexagonal 6H-perovskite structure (space group P6(3)/mmc). This phase, when heated at high temperature under a stream of Ar gas, transforms to an oxygen-deficient phase delta = 0.55), which is an ordered cubic perovskite structure (space group Fm-3m). The crystallographic and magnetic properties of the obtained phases are compared, and it is clear that the magnetic properties are significantly affected by oxygen non-stoichiometry. These changes of magnetic properties for such a slight decrease in oxygen content are interpreted as a result of structural transformations. Together with the experimental results based on neutron powder diffraction data a discussion of some aspects of the structural transformation (P6(3)/mmc -> Fm-3m) is presented.
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