Incommensurate Modulations and Perovskite Growth in LaxSr2-xMnO4−δ Affecting Solid Oxide Fuel Cell Conductivity

Ruddlesden-Popper LaxSr2−xMnO4−δ materials are interesting symmetric solid oxide fuel cell electrodes due to their good redox stability, mixed ionic and electronic conducting behavior and thermal expansion that matches well with common electrolytes. In reducing environments - as at a solid oxide fuel cell anode - the x = 0.5 member, i.e. La0.5Sr1.5MnO4−δ, has a much higher total conductivity than compounds with a different La/Sr ratio, although all those compositions have the same K2NiF4-type I4/mmm structure. The origin for this conductivity difference is not yet known in literature. Now, a combination of in-situ and ex-situ 3D electron diffraction, high-resolution imaging, energy-dispersive X-ray analysis and electron energy-loss spectroscopy uncovered clear differences between x=0.25 and x=0.5 in the pristine structure, as well as in the transformations upon high-temperature reduction. In La0.5Sr1.5MnO4−δ, Ruddlesden-Popper n=2 layer defects and an amorphous surface layer are present, but not in La0.25Sr1.75MnO4−δ. After annealing at 700°C in 5% H2/Ar, La0.25Sr1.75MnO4−δ transforms to a tetragonal 2D incommensurately modulated structure with modulation vectors q1 = 0.2848(1) · (a* +b*) and q2 =0.2848(1) · (a* -b*), whereas La0.5Sr1.5MnO4−δ only partially transforms to an orthorhombic 1D incommensurately modulated structure, with q = 0.318(2) · (a* - b*). Perovskite domains grow at the crystal edge at 700°C in 5% H2 or vacuum, due to the higher La concentration on the surface compared to the bulk, which leads to a different thermodynamic equilibrium. Since it is known that a lower degree of oxygen vacancy ordering and a higher amount of perovskite blocks enhance oxygen mobility, those differences in defect structure and structural transformation upon reduction, might all contribute to the higher conductivity of La0.5Sr1.5MnO4−δ in solid oxide fuel cell anode conditions compared to other La/Sr ratios.