b S Supporting Information T he relation of surface cation chemistry and surface electronic structure to oxygen reduction reaction (ORR) kinetics remains an outstanding question to this day in the search for highly active cathodes for solid oxide fuel cells (SOFCs). While traditionally perovskite-type transition-metal oxides have been extensively investigated as SOFC cathodes, 1,2 more recent studies highlight the potential of layered oxide cathodes. 3À5 On the surface of the perovskite-structured La 1Àx Sr x MnO 3 , a widely used and studied SOFC cathode material, 1,2,6À8 the fractional presence of constituent cations can deviate from the nominal bulk stoichiometry significantly 9À13 because of an enrichment of Sr or La cations on the surface. It has been possible to control the bulk magnetic and electronic properties of perovskite thin films by manipulating their lattice parameters with different growth conditions, hydrostatic pressure, or use of substrates with a different lattice mismatch to the films. 14À18 Furthermore, the impact of the lattice strain on the surface electronic structure and reactivity has been long demonstrated for low-temperature noble metal electrocatalysts. 19,20 On the other hand, the role of lattice strain on the surface cation and anion chemistry, electronic structure, and ionic transport, which all influence the ORR activity of SOFCrelated oxides, is attracting its due interest only recently.We have recently demonstrated, from first principles-based calculations, that the epitaxial strain up to a critical tensile strain value favors oxygen-vacancy formation as well as oxygen adsorption on another widely studied SOFC cathode, LaCoO 3 . 21 Experiments validating the direct role of strain on the reactivity with oxygen and oxygen transport in SOFC materials have been yet scarce. Sase et al. showed that the oxygen surface exchange rate at the heterointerface of La 0.6 Sr 0.4 CoO 3 /(La,Sr) 2 CoO 4 thin films is larger by three orders of magnitude compared with the single-phase cobaltite surfaces. 22 A reasonable hypothesis that could explain the enhanced oxygen exchange at that interface region is the role of local strains. Studies on fluorite systems have suggested strong coupling of biaxial strain also to the oxygen ion diffusion. 23À25 In this Letter, we report our results, interpreted in light of our first principles-based simulations, on the strain-induced changes in the surface chemical and electronic state of La 0.7 Sr 0.3 MnO 3 (LSM) as a model system. We assessed two key parameters for reactivity with oxygen as a function of strain: (1) chemical environment on the LSM surface, in particular, the segregation of Sr cations and oxygen vacancy formation, experimentally probed with angleresolved X-ray photoelectron spectroscopy and computationally assessed through segregation/formation energy calculations and (2) surface electronic structure, experimentally probed using scanning tunneling microscopy and spectroscopy (both at ambient and in situ at elevated temperatures) and computat...

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Pulsed laser deposition was used to grow Sr(2)FeMoO(6) films of different thicknesses on MgO(100), SrTiO(3)(100), and LaAlO(3)(100) with respective lattice mismatches of +6.2%, -1.2%, and -4.3%. Surface roughness and morphology, and film crystal quality and epitaxy were determined by atomic force microscopy and x-ray diffraction, respectively. Two-dimensional layer-by-layer growth was evident for the Sr(2)FeMoO(6) grown on MgO and SrTiO(3) with the film becoming smoother with increasing thickness. The Sr(2)FeMoO(6) films had more nucleation sites on MgO than SrTiO(3). On LaAlO(3), however, three-dimensional progressive growth of flakelike Sr(2)FeMoO(6) nanostructures was observed for all film thicknesses. High-resolution x-ray diffraction measurements indicated that the Sr(2)FeMoO(6) films are near-epitaxial and c-axis oriented on all the substrates. Reciprocal space maps further revealed that Sr(2)FeMoO(6) grows on MgO with relatively constant lattice parameters with increasing film thickness. For films thicker than 120 nm, the formation of a second phase was observed on SrTiO(3) and LaAlO(3) but not on MgO, suggesting that the formation of a second phase provides an effective strain relief in the former. These results suggested a different growth mechanism for the Sr(2)FeMoO(6) films on MgO compared to the SrTiO(3) and LaAlO(3) substrates.

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Single-phase CrO 2 nanostructured thin films have been grown directly on MgO͑100͒ by pulsed laser ablation of a metallic Cr target in an O 2 environment. X-ray diffraction shows that these films are strained and consist of CrO 2 crystallites with two possible epitaxial relationships to the substrate; either CrO 2 ͑110͒ or CrO 2 ͑200͒ is parallel to MgO͑100͒. Scanning electron microscopy and atomic force microscopy reveal orthogonally arranged nanoneedles and platelike structures ͑both 30-50 nm thick͒. X-ray photoemission confirms that the films are primarily CrO 2 covered with a thin CrO 3 overlayer and indicates its complete synthesis without any residual metallic Cr.