PEROVSKITES FOR SOLID OXIDE FUEL CELLS. Aniceta Skowron, Pengnian Huang and Anthony Petric, Department of Material Science and Engineering, McMaster University, Hamilton, Ontario, L8S 4L7 Canada

We have studied the structure and conductivity of perovskite-type La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825, the new, superior oxygen ion conductor (1, 2) for use as electrolyte in high temperature solid oxide fuel cells. Since the transport properties of substitutionally disordered and oxygen deficient materials are largely determined by their local structure, which may differ in symmetry from the average long range crystalline symmetry, we studied the local and average structures of La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825 using powder neutron and electron diffraction and high resolution lattice imaging in TEM. Despite the macroscopic cubic symmetry revealed by neutron powder diffraction, La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825 exhibits a lower symmetry short range order evidenced by, firstly, weak and sharp superstructure reflections and, secondly, streaks of diffuse scattering in the electron diffraction patterns. Neutron powder profile refinement shows that the oxygen nuclei are displaced from the sites of cubic symmetry in a manner similar to that in perovskite-related layered structure of brownmillerite. Thermal stability of the local structures was studied by high temperature electron diffraction and will be discussed in relation with the conductivity measurements. Modeling of the oxygen mobility was performed using the bond valence method (3).

References

1. Ishihara,T., Matsuda,H. and Takita,Y., J. Am. Chem. Soc., 116, 3801, 1994.

2. Feng, M. and Goodenough, J.B., Europ. J. of Solid State Chem., 31, 663, 1994

3. Brown, I.D., Solid State Ionics, 31,203, 1988.