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

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1.
  • 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.
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2.
  • 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.
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3.
  • Eriksson, Annika K., 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.
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