Molecular dynamics study of oxygen diffusion in Pr₂NiO_4+δ

Oxygen transport in tetragonal Pr2NiO4+δ has been investigated using molecular dynamics simulations in conjunction with a set of Born model potentials. Oxygen diffusion in Pr2NiO4+δ 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 K2NiF4 structure materials

Anisotropic oxygen diffusion in tetragonal La₂NiO_4+δ: molecular dynamics calculations

Molecular dynamics simulations, used in conjunction with a set of Born model potentials, have been employed to study oxygen transport in tetragonal La2NiO4+δ. 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 La2NiO4+δ

Interstitialcy diffusion of oxygen in tetragonal La₂CoO_4+δ

We report on the mechanism and energy barrier for oxygen diffusion in tetragonal La2CoO4+δ. 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 La2CoO4+δ. 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 principles-based 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 CoO6 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

Unveiling the outstanding oxygen mass transport properties of Mn-rich perovskites in grain boundary-dominated La0.8Sr0.2(Mn1-xCox)0.85O3-d nanostructures

Ion transport in solid-state devices is of great interest for current and future energy and information technologies. A superior enhancement of several orders of magnitude of the oxygen diffusivity has been recently reported for grain boundaries in lanthanum strontium manganites. However, the significance and extent of this unique phenomenon is not yet established. Here, we fabricate a thin film continuous composition map of the La0.8Sr0.2(Mn1-xCox)0.85O3-d family revealing a substantial enhancement of the grain boundary oxygen mass transport properties for the entire range of compositions. Through isotope-exchange depth profiling coupled to secondary ion mass spectroscopy, we show that this excellent performance is not directly linked to the bulk of the material but to the intrinsic nature of the grain boundary. In particular, the great increase of the oxygen diffusion in Mn-rich compositions unveils an unprecedented catalytic performance in the field of Mixed Ionic Electronic Conductors. These results present grain boundaries engineering as a novel strategy for designing highly performing materials for solid state ionics based devices

Towards comprehensive assessment of mitral regurgitation using cardiovascular magnetic resonance

Cardiovascular magnetic resonance (CMR) is increasingly used to assess patients with mitral regurgitation. Its advantages include quantitative determination of ventricular volumes and function and the mitral regurgitant fraction, and in ischemic mitral regurgitation, regional myocardial function and viability. In addition to these, identification of leaflet prolapse or restriction is necessary when valve repair is contemplated. We describe a systematic approach to the evaluation of mitral regurgitation using CMR which we have used in 149 patients with varying etiologies and severity of regurgitation over a 15 month period

Correlating Surface Crystal Orientation and Gas Kinetics in Perovskite Oxide Electrodes

Solid–gas interactions at electrode surfaces determine the efficiency of solid-oxide fuel cells and electrolyzers. Here, the correlation between surface–gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system LaSrCoFeO. The gas-exchange kinetics are characterized by synthesizing epitaxial half-cell geometries where three single-variant surfaces are produced [i.e., LaSrCoFeO/LaSrGaMgO/SrRuO/SrTiO (001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface-orientation dependency of the gas-exchange kinetics, wherein (111)-oriented surfaces exhibit an activity >3-times higher as compared to (001)-oriented surfaces. Oxygen partial pressure ((Formula presented.))-dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas-exchange kinetics, the reaction mechanisms and rate-limiting steps are the same (i.e., charge-transfer to the diatomic oxygen species). First-principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron-based, ambient-pressure X-ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin-film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)-surface exhibits a high density of active surface sites which leads to higher activity.R.G. and A.F. contributed equally to this work. R.G., A.F., A.L., T.C., E.E., and L.W.M. acknowledge the support of the National Science Foundation under Grant OISE-1545907. D.P. acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 797123. G.V. acknowledges the support of the National Science Foundation under Grant DMR-1708615. V.T. and T.I. acknowledge financial support from a Grant-in-Aid for Specially Promoted Research No. 16H06293) from MEXT, Japan and through the Japan Society for the Promotion of Science and the Solid Oxide Interfaces for Faster Ion Transport JSPS Core-to-Core Program (Advanced Research Networks). J.K. acknowledge the support by World Premier International Research Center Initiative (WPI), Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT), Japan, Solid Oxide Interfaces for Faster Ion Transport (SOIFIT) JSPS/EPSRC (EP/P026478/1) Core-to-Core Program (Advanced Research Networks). This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231

Oxygen-vacancy ordering in lanthanide-doped ceria: Dopant-type dependence and structure model

Studies of electron energy loss spectroscopy and selected area electron diffraction (SAED) were systematically performed on 15 and 25 at. % lanthanide (Ln)-doped ceria samples (Ln=Sm, Gd, Dy, and Yb), through which the local ordering of oxygen vacancies that develops with increase in doping level was confirmed in the sequence of (Gd,Sm)>Dy>Yb. Furthermore, a monotone correlation between the development of the ordering and the degradation of ionic conductivity with increasing the doping concentration from 15 to 25 at. % was observed. Based on the analysis of SAED patterns, a structural model for the ordering of oxygen vacancies has been constructed, in which the arrangement of oxygen vacancies is similar to that in C-type Ln2O3 oxides and the 110 pairs of the vacancies are preferred. Then, the factors that can influence the formation of the ordering are discussed