ORIGINAL PAPER Long-term structural surface modifications of mixed conducting La 2 NiO 4+d at high temperatures Nicolas Gauquelin Æ Thomas E. Weirich Æ Monica Ceretti Æ Werner Paulus Æ Michael Schroeder Received: 24 December 2008 / Accepted: 19 March 2009 / Published online: 31 March 2009 Ó Springer-Verlag 2009 Abstract Long-term annealing of La 2 NiO 4?d single crystals at 1,273 K in air leads to the formation of nickel- rich Ruddlesden–Popper phases at the single-crystal sur- faces. Both the composition and the morphology of these phases depend on the surface orientation; whereas only crystallites of La 4 Ni 3 O 10-d were observed on (001) sur- faces, both La 3 Ni 2 O 7-d and La 4 Ni 3 O 10-d were formed on (100) surfaces. The formation of the nickel-rich RP phases is believed to be due to surface segregation of nickel or evaporation of a volatile lanthanum species. The crys- tallographic (001) planes inside the La 3 Ni 2 O 7-d and La 4 Ni 3 O 10-d crystallites were found to be oriented in the direction of preferential crystallite growth, which indicates that the diffusion of lanthanum and nickel cations is faster along the crystallographic (001) planes than perpendicular to these planes. Keywords Lanthanum nickel oxides Á Mixed conductor Á Electron microscopy Á Electron diffraction Á Membranes Introduction Mixed conducting materials based on lanthanum nickel oxides, such as La 2 NiO 4?d , have received much interest due to their potential use as cathode materials in solid oxide fuel cells (SOFC) and as oxygen separation membranes. La 2 NiO 4?d appears promising for SOFC cathodes because it combines acceptable electronic conductivity [1, 2] with reasonable oxide–ion conductivity at intermediate temper- atures. Operation as an oxygen-permeable membrane, however, imposes an additional constraint on the material, since such membranes are often exposed to a gas stream containing reducing or reactive gases. For example, in an oxycoal power plant process [3], the membrane material would be in contact with recirculated flue gas, which con- tains CO 2 as the major component. So far, most oxides that have been identified as potential membrane materials are perovskite-structured mixed conductors whose excellent oxygen permeabilities rely on alkaline-earth element doping (e.g., Ba 1 - x Sr x Co 1-y Fe y O 3?d [4]). In contact with CO 2 , these oxides suffer from long-term degradation because the alkali-earth elements form carbonates that are stable even at typical membrane operation temperatures [5–8]. On the other hand, the lanthanum nickelates consti- tute a class of alkaline-earth-free materials that may be less susceptible to degradation. Lanthanum nickelates form a series of Ruddlesden–Popper (RP)-type phases with general formula LaO(LaNiO 3 ) n . The perovskite-structured com- pound LaNiO 3 represents the end member of this series (n = ?), whereas the first three members, La 2 NiO 4?d , La 3 Ni 2 O 7-d (n = 2) and La 4 Ni 3 O 10-d (n = 3), can be considered layered perovskites whose structures are formed by alternating the stacking of single LaO layers (rock–salt structure) and n consecutive LaNiO 3 layers (perovskite structure) along the crystallographic c direction [9–19]. N. Gauquelin Á M. Schroeder (&) Institute of Physical Chemistry, RWTH Aachen, Aachen, Germany e-mail: schroeder@rwth-aachen.de T. E. Weirich Central Facility for Electron Microscopy, RWTH Aachen, Aachen, Germany M. Ceretti Á W. Paulus University of Rennes 1, UMR CNRS 6226, Rennes, France 123 Monatsh Chem (2009) 140:1095–1102 DOI 10.1007/s00706-009-0146-2