Solid oxide fuel cells are of technological interest as they offer high efficiency for energy conversion in a clean way. Understanding fundamental aspects of oxygen self-diffusion in solid state ionic systems is important for the discovery of next-generation electrolyte and cathode material compositions and microstructures that can enable the operation of SOFCs at lower temperatures more efficiently, durably, and economically. In the present perspective article, we illustrate the important role of modelling and simulations in providing direct atomic scale insights on the oxygen ion transport mechanisms and conduction properties in the cathode and electrolyte materials, and in accelerating the progress from old materials to new concepts. We first summarize the ionic transport mechanisms in the traditional cathode and electrolyte materials which have been widely studied. We then pay our attention to the non-traditional materials and their oxygen transport paths from recent studies, focusing on structural and transport anisotropy and lattice dynamics. Lastly, we highlight the new developments in the potential to increase the ionic conductivity of the traditional materials through external mechanical stimuli, bringing about the mechano-chemical coupling to drive fast ionic transport.

Molecular dynamics simulations, used in conjunction with a set of Born model potentials, have been employed to study oxygen transport in tetragonal La 2 NiO 4+d . We predict an interstitialcy mechanism with an activation energy of migration of 0.51 eV in the temperature range 800-1100 K. The simulations are consistent with the most recent experiments. The prevalence of oxygen diffusion in the a-b plane accounts for the anisotropy observed in measurements of diffusivity in tetragonal La 2 NiO 4+d .

Oxygen transport in tetragonal Pr 2 NiO 4+d has been investigated using molecular dynamics simulations in conjunction with a set of Born model potentials. Oxygen diffusion in Pr 2 NiO 4+d is highly anisotropic, occurring almost entirely via an interstitialcy mechanism in the a-b plane. The calculated oxygen diffusivity has a weak dependence upon the concentration of oxygen interstitials, in agreement with experimental observations. In the temperature range 800-1500 K, the activation energy for migration varied between 0.49 and 0.64 eV depending upon the degree of hyperstoichiometry. The present results are compared to previous work on oxygen self-diffusion in related K 2 NiF 4 structure materials.

Electronic structure calculations are used to predict the activation enthalpies of diffusion for a range of impurity atoms ͑aluminium, gallium, indium, silicon, tin, phosphorus, arsenic, and antimony͒ in germanium. Consistent with experimental studies, all the impurity atoms considered diffuse via their interaction with vacancies. Overall, the calculated diffusion activation enthalpies are in good agreement with the experimental results, with the exception of indium, where the most recent experimental study suggests a significantly higher activation enthalpy. Here, we predict that indium diffuses with an activation enthalpy of 2.79 eV, essentially the same as the value determined by early radiotracer studies. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2918842͔ Germanium ͑Ge͒ has the potential to replace silicon ͑Si͒ in advanced nanoelectronic devices because of the higher mobility of holes and electrons, compatibility with Si manufacturing processes, increased dopant solubility, and smaller band gap. 1 The precise control required for the fabrication of these devices would be greatly aided by an accurate determination of the diffusion properties of impurities in Ge. 2 This is particularly important for donor impurities for which activation control can be problematic. 3 In previous studies, it has been concluded that most impurities mainly occupy substitutional lattice sites in Ge and, with the exception of boron ͑B͒, dopant diffusion is mainly mediated by vacancies ͑V͒ as interstitial mechanisms typically have significantly higher activation enthalpies. 2, Aluminium ͑Al͒, gallium ͑Ga͒, indium ͑In͒, and B are acceptor atoms that can potentially be used as p-type dopants in Ge technology. Recent experiments 4 on B diffusion in Ge yield an activation enthalpy of 4.65 eV that agrees with earlier results, but the absolute values of the B diffusion coefficients are several orders of magnitude lower than those reported earlier. 5 Previous experimental studies on Al diffusion yield activation enthalpies in the range of 3.2-3.45 eV. 6,7 Södervall et al. 8 obtained an activation enthalpy of 3.31 eV for Ga diffusion in Ge. This value is supported by the more recent experimental studies of Riihimäki et al. 9 who determined an activation enthalpy of 3.21 eV for Ga diffusion in Ge via a V-mediated mechanism. The spread in the data reported for the activation enthalpy of In diffusion in Ge is especially large ͑0.85 eV͒. The radiotracer studies of Pantaleev 10 suggest a value of 2.78 eV, whereas the In profiles measured by Dorner et al. 11 by means of secondary ion mass spectrometry ͑SIMS͒ yield a value of 3.63 eV.Carbon ͑C͒, Si, and tin ͑Sn͒ are important isovalent impurities. C atoms have been observed to be relatively immobile; however, they can retard the diffusion of phosphorus ͑P͒, arsenic ͑As͒, and antimony ͑Sb͒ atoms in Ge. 26,27 Recent experimental studies by Silvestri et al. 15 ͑using SIMS͒ concluded that Si diffusion in Ge is mediated by V with an activation enthalpy of 3.32 eV, whereas previous studies ...

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We report on the mechanism and energy barrier for oxygen diffusion in tetragonal La 2 CoO 4+d . The first principles-based calculations in the Density Functional Theory (DFT) formalism were performed to precisely describe the dominant migration paths for the interstitial oxygen atom in La 2 CoO 4+d . Atomistic simulations using molecular dynamics (MD) were performed to quantify the temperature dependent collective diffusivity, and to enable a comparison of the diffusion barriers found from the force field-based simulations to those obtained from the first principlesbased calculations. Both techniques consistently predict that oxygen migrates dominantly via an interstitialcy mechanism. The single interstitialcy migration path involves the removal of an apical lattice oxygen atom out from the LaO-plane and placing it into the nearest available interstitial site, whilst the original interstitial replaces the displaced apical oxygen on the LaO-plane. The facile migration of the interstitial oxygen in this path is enabled by the cooperative titling-untilting of the CoO 6 octahedron. DFT calculations indicate that this process has an activation energy significantly lower than that of the direct interstitial site exchange mechanism. For 800-1000 K, the MD diffusivities are consistent with the available experimental data within one order of magnitude. The DFT-and the MD-predictions suggest that the diffusion barrier for the interstitialcy mechanism is within 0.31-0.80 eV. The identified migration path, activation energies and diffusivities, and the associated uncertainties are discussed in the context of the previous experimental and theoretical results from the related Ruddlesden-Popper structures.

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