Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
Phys Chem Chem Phys. 2011 Feb 14;13(6):2242-9. doi: 10.1039/c0cp01603a. Epub 2010 Dec 6.
We report on the mechanism and energy barrier for oxygen diffusion in tetragonal La(2)CoO(4+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 La(2)CoO(4+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 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.
我们报告了四方相 La(2)CoO(4+δ)中氧扩散的机制和能垒。采用基于密度泛函理论（DFT）的第一性原理计算，精确描述了 La(2)CoO(4+δ)中间隙氧原子的主要迁移路径。使用分子动力学（MD）进行原子模拟，以量化温度相关的集体扩散率，并能够将基于力场的模拟得出的扩散势垒与第一性原理计算得出的扩散势垒进行比较。这两种技术都一致表明，氧主要通过间隙机制迁移。单个间隙迁移路径涉及从 LaO 平面中移除一个顶点晶格氧原子，并将其放置在最近的可用间隙位置，而原始的间隙则取代 LaO 平面上的被取代的顶点氧。在这个路径中，间隙氧的易迁移是由 CoO(6)八面体的协同倾斜-倾斜实现的。DFT 计算表明，这个过程的激活能明显低于直接间隙位交换机制。在 800-1000 K 之间，MD 扩散率与可用的实验数据在一个数量级内一致。DFT 和 MD 的预测表明，间隙机制的扩散势垒在 0.31-0.80 eV 范围内。所确定的迁移路径、激活能和扩散率以及相关的不确定性，都在与相关的 Ruddlesden-Popper 结构的先前实验和理论结果的背景下进行了讨论。
