Polaron size and shape effects on oxygen vacancy interactions in lanthanum strontium ferrite
Polaron size and shape effects on oxygen vacancy interactions in lanthanum strontium ferrite
复制标题
极化子尺寸和形状对镧锶铁氧体中氧空位相互作用的影响
DOI：
10.1039/c7ta06948k
复制
发表时间：
2017
影响因子：
11.9
通讯作者：
Qi, Yue
中科院分区：
文献类型：
--
作者：
Das, Tridip;Nicholas, Jason D.;Qi, Yue
异价掺杂和多价晶格离子的电荷状态决定了混合离子和电子导体（MIECs）的氧非化学计量比（δ）。不幸的是，它一直是具有挑战性的建模和实验，以确定多价离子的电荷状态在MIECs。在这里，使用自旋极化密度泛函理论（DFT）预测的Fe磁矩，确定了MIEC La 1 −xSrxFeO3−δ（LSF）的各种组成和相的Fe电荷态分布。结果发现，正方体配位的氧空位相邻的Fe原子和八面体配位的氧空位距离的Fe原子的Fe 3d轨道之间的电子占据和晶体场分裂诱导的差异决定了氧空位形成期间产生的过量电子是否保持在第一最近邻Fe原子处（导致小的氧空位极化子，如在LaFeO3中）或分布到第二近邻的Fe原子（导致大的氧空位极化子，如在SrFeO3中）。随着Sr的增加，极化子尺寸和各向异性形状的变化逐渐增大，导致氧空位相互作用增加，如氧空位形成能在临界δ阈值以上增加所示。这与实验结果一致，实验结果表明，富Sr的LSF和高度缺氧的组合物是易于氧空位有序诱导的相变，而贫Sr和富氧的LSF组合物不是。由于氧空位诱导的相变导致移动的氧空位位点分数（X）的降低，因此对于多种LSF组合物和相，使用组合的热力学和DFT方法预测δ和X两者作为温度和氧分压的函数。
Both aliovalent doping and the charge state of multivalent lattice ions determine the oxygen non-stoichiometry (δ) of mixed ionic and electronic conductors (MIECs). Unfortunately, it has been challenging for both modeling and experiments to determine the multivalent ion charge states in MIECs. Here, the Fe charge state distribution was determined for various compositions and phases of the MIEC La1−xSrxFeO3−δ (LSF) using the spin-polarized density functional theory (DFT)-predicted magnetic moments on Fe. It was found that electron occupancy and crystal-field-splitting-induced differences between the Fe 3d-orbitals of the square pyramidally coordinated, oxygen-vacancy-adjacent Fe atoms and the octahedrally-coordinated, oxygen-vacancy-distant-Fe atoms determined whether the excess electrons produced during oxygen vacancy formation remained localized at the first nearest neighbor Fe atoms (resulting in small oxygen vacancy polarons, as in LaFeO3) or were distributed to the second-nearest-neighbor Fe atoms (resulting in large oxygen vacancy polarons, as in SrFeO3). The progressively larger polaron size and anisotropic shape changes with increasing Sr resulted in increasing oxygen vacancy interactions, as indicated by an increase in the oxygen vacancy formation energy above a critical δ threshold. This was consistent with experimental results showing that Sr-rich LSF and highly oxygen deficient compositions are prone to oxygen-vacancy-ordering-induced phase transformations, while Sr-poor and oxygen-rich LSF compositions are not. Since oxygen vacancy induced phase transformations cause a decrease in the mobile oxygen vacancy site fraction (X), both δ and X were predicted as a function of temperature and oxygen partial pressure, for multiple LSF compositions and phases using a combined thermodynamics and DFT approach.