Oxygen Diffusion and Structural Distortion in Non-stoichiometric Oxides
Abstract
Solid state fuel cells (SOFC) will play key role in green technologies as they are efficiently producing electricity with low emissions. Our consortium (PSI-Montpellier) investigates the complex structural behavior of La2CoO4+x, La2-xSrxCuO4+y and related materials as this materials show oxygen diffusion at moderate temperatures. The tilting of the MO6 octahedra is playing here a key role, but also yielding complicated modulations and unit cells up to 25’000 Å. We are using combined X-ray and neutron diffraction methods as well as macroscopic measurements such as O18-back exchange and secondary ion mass spectroscopy (SIMS). We work in a consortium between PSI, Montpellier (France), Torino (Italy) and ILL (Grenoble).
Project details
Solid state oxygen fuel cells (SOFC) stimulate the research on solid oxygen ion conductors, as efficient production of electric power is a key issue in green technologies.
We have seen a renaissance concerning the research activities of oxides with perovskite structure or related structures such as Ruddlesden-Popper phases (Amow and Skinner 2006) and Brownmillererites (Ceretti, Piovano et al. 2012) as these materials show two properties simultaneously: oxygen conduction at moderate temperatures and high stability (oxide). These activities began in 1987 with the high temperature superconductors (Bednorz and Müller 1986) and were prolonged by studies focused on the giant magnetoresistivity and on compounds showing the so-called spin ladders as structural and electronic characteristic element. For all these phases, a variation of the oxygen stoichiometry strongly influences the physical properties.
Our consortium (PSI-Montpellier) has shown that the complex structural behavior of La2CoO4+x with the oxygen stoichiometry, in detail discussed in a recent thesis work by Loïc Le Dréau (Le Dréau 2011, Le Dréau, Paulus et al. 2012) is related to oxygen diffusion. As stoichiometric La2CoO4 crystallizes at ambient temperature in the Bmab space group, showing a similar CoO6 tilting arrangement as observed for La2CuO4, already a small amount of oxygen intercalation leads to an averaged structure in Fmmm space group. This means that with the oxygen uptake the local distortions between interstitial and apical oxygen atoms destroy on one hand the 3D order of the CoO6 tilting scheme, but – on the other hand - maintain the orthorhombic symmetry. Although the structure is always disordered for small and medium quantities of intercalated oxygen atoms, this is no longer true for stoichiometries beyond x > 0.2.
Our consortium (PSI-Montpellier) has shown that the complex structural behavior of La2CoO4+x with the oxygen stoichiometry, in detail discussed in a recent thesis work by Loïc Le Dréau (Le Dréau 2011, Le Dréau, Paulus et al. 2012) is related to oxygen diffusion. As stoichiometric La2CoO4 crystallizes at ambient temperature in the Bmab space group, showing a similar CoO6 tilting arrangement as observed for La2CuO4, already a small amount of oxygen intercalation leads to an averaged structure in Fmmm space group. This means that with the oxygen uptake the local distortions between interstitial and apical oxygen atoms destroy on one hand the 3D order of the CoO6 tilting scheme, but – on the other hand - maintain the orthorhombic symmetry. Although the structure is always disordered for small and medium quantities of intercalated oxygen atoms, this is no longer true for stoichiometries beyond x > 0.2.
References
- Ceretti, M., A. Piovano, A. Cousson, T. Berthier, M. Meven, G. Agostini, J. Schefer, O. Hernandez, C. Lamberti and W. Paulus (2012), "Growth and characterization of large high quality brownmillerite CaFeO2.5 single crystals", CrystEngComm, 14: 5771–5776.
- Le Dréau, L. (2011), "Phase transistions and oxygen ordering in La2CoO4+d and (T,T') La2CuO4: Single cyrstal growth and structural studies using synchrotron and neutron diffraction methods", Thesis, University of Rennes 4366: 1-258 (available as LNS-report 241).
- Le Dréau, L., W. Paulus, J. Schefer, M. Ceretti and C. Prestipino (2012), "Oxygen diffusion and structural properties in LaCoO4+x", Inorganic Chemistry accepted.
- Villesuzanne, A., W. Paulus, A. Cousson, S. Hosoya, L. Le Dréau, O. Hernandez, C. Prestipino, M. I. Houchati and J. Schefer (2011), "On the role of lattice dynamics on low-temperature oxygen mobility in solid oxides: a neutron diffraction and first-principles investigation of La2CuO4+d", J Solid State Electrochem 15: 357-366.
- Le Toquin, R., W. Paulus, A. Cousson, C. Prestipino and C. Lamberti (2006), "Time-Resolved in Situ Studies of Oxygen Intercalation into SrCoO2.5, Performed by Neutron Diffraction and X-ray Absorption Spectroscopy", J. Am. Chem. Soc., 128: 13161–13174.
- Amow, G. and S. J. Skinner (2006). "Recent developments in Ruddlesden-Popper-nickelate systems for solid oxide fuel cell cathodes", J. Solid State Electrochem. 10: 538-546.
- Bednorz, J. G. and K. A. Müller (1986), "Possible high Tc superconductivity in the Ba-La-Cu-O system", Zeitschrift für Physik B 64: 189-193.
Funding: Others
Partners:
Prof. W. Paulus, M. Ceretti, Chimie et Cristallochimie de Matériaux (C2M), Université de Montpellier 2, France
Contact:
Lukas Keller
lukas.keller@psi.ch