Oxide interfaces with enhanced ion conductivity
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Introduction The recent surge of interest in oxide interfaces has been fueled by the possibility of stabilizing electronic ground states, leading to emerging properties. Many of these complex oxides share a common perovskite structure with similar lattice parameters that allow the growth of epitaxial interfaces with high structural perfection. At interfaces, electron correlations establish novel forms of coupling between electronic ground states of the adjoining materials, which have been proposed to be the origin of emergent properties.1 A paradigmatic case is the metallic state at the interface between two band insulators as in LaAlO3/ SrTiO3 (LAO/STO) heterostructures.2 This finding triggered a new research field aimed at finding interesting novel phases at oxide interfaces with a high potential for applications.3–5 At interfaces, the important quantities controlling the nucleation of different electronic phases (charge density, electrostatic repulsion and bandwidth) may change substantially in a phenomenon called electronic reconstruction.6,7 In addition, interface strain is also an important parameter controlling phase stabilization.8–13 Oxides, as compared to other materials, are able to accommodate very large amounts of epitaxial strain without breaking into islands or structural domains. This is probably due to the large polarizability of the oxygen sublattice,7 which admits quite large deformations in the form of rotations and distortions of the oxygen octahedra.
The possibility of tailoring electronic properties at oxide interfaces has raised the question of whether the properties of oxygen ion conducting materials could also be modified at interfaces. In this regard, the coherent growth of strained interfaces in heterostructures combining materials with different degrees of lattice mismatch has been shown to promote ion diffusivity14 and thus, these heterostructures may play an important role in the optimization of materials for electrochemical devices such as batteries and fuel cells that are key to commercially viable low power energy generation and storage.15,16 In particular, enhanced oxide-ion conductivity at oxide interfaces would be relevant for lowering the operation temperature of solid oxide fuel cells (SOFCs). In SOFCs, O2– ions form at the cathode and diffuse through a solid electrolyte material at elevated temperatures (usually 800–1000°C) to react with H+ ions in the anode to produce water. The high operation temperatures favor internal fuel reformation, electrode processes, and ionic migration through the electrolyte, but impose serious restrictions on materials selection due to thermal stress or fatigue. Thus, a major materials research goal today is to reduce the operating temperature of SOFCs without compromising device performance.15 Novel electrolytes are needed with higher oxide-ion conductivity and also electrode materials with higher oxygen exchange rates.
Carlos Leon, Department of Applied Physics, Universidad Complutense, Madrid; carlos.leon@fis.ucm.es Jacobo Santamaria, Applied Phy
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