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

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1.
  • Andreasson, Jakob, 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.
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2.
  • 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.
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3.
  • 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.
4.
  • Norberg, Stefan T., et al. (författare)
  • Local structural properties of 0.5BiMnO(3)-0.5ATiO(3) (A = Ba or Sr)
  • 2010
  • Ingår i: Chemical Communications. - 1359-7345. ; 46:9, s. 1455-1457
  • Tidskriftsartikel (refereegranskat)abstract
    • Neutron total scattering data have been used to probe the long-and short-range structure of 0.5BiMnO(3)-0.5ATiO(3) (A = Ba or Sr). Modelling of the total scattering data, using the reverse Monte Carlo (RMC) method, reveals local disorder that differs substantially from the average structure. Significantly, the Bi3+ cations prefer to adopt an asymmetric coordination due to the influence of the lone-pair electrons.
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