Parkes Michael A, Refson Keith, d'Avezac Mayeul, Offer Gregory J, Brandon Nigel P, Harrison Nicholas M
†Department of Earth Science and Engineering, Imperial College London, London SW7 2BP, U.K.
‡Thomas Young Centre, Department of Chemistry, Imperial College London, London SW7 2AZ, U.K.
J Phys Chem A. 2015 Jun 18;119(24):6412-20. doi: 10.1021/acs.jpca.5b02031. Epub 2015 May 29.
Yttria-stabilized zirconia (YSZ) is an important oxide ion conductor with applications in solid oxide fuel cells (SOFCs) and oxygen sensing devices. Doping the cubic phase of zirconia (c-ZrO2) with yttria (Y2O3) is isoelectronic, as two Zr(4+) ions are replaced by two Y(3+) ions, plus a charge compensating oxygen vacancy (Ovac). Typical doping concentrations include 3, 8, 10, and 12 mol %. For these concentrations, and all below 40 mol %, no phase with long-range order has been observed in either X-ray or neutron diffraction experiments. The prediction of local defect structure and the interaction between defects is therefore of great interest. This has not been possible to date as the number of possible defect topologies is very large and to perform reliable total energy calculations for all of them would be prohibitively expensive. Previous theoretical studies have only considered a selection of representative structures. In this study, a comprehensive search for low-energy defect structures using a combined classical modeling and density functional theory approach is used to identify the low-energy isolated defect structures at the dilute limit, 3.2 mol %. Through analysis of energetics computed using the best available Born-Mayer-Huggins empirical potential model, a point charge model, DFT, and a local strain energy estimated in the harmonic approximation, the main chemical and physical descriptors that correlate to the low-energy DFT structures are discussed. It is found that the empirical potential model reproduces a general trend of increasing DFT energetics across a series of locally strain relaxed structures but is unreliable both in predicting some incorrect low-energy structures and in finding some metastable structures to be unstable. A better predictor of low-energy defect structures is found to be the total electrostatic energy of a simple point charge model calculated at the unrelaxed geometries of the defects. In addition, the strain relaxation energy is estimated effectively in the harmonic approximation to the imaginary phonon modes of undoped c-ZrO2 but is found to be unimportant in determining the low-energy defect structures. These results allow us to propose a set of easily computed descriptors that can be used to identify the low-energy YSZ defect structures, negating the combinatorial complexity and number of defect structures that need to be considered.
钇稳定氧化锆（YSZ）是一种重要的氧化物离子导体，应用于固体氧化物燃料电池（SOFC）和氧传感装置。用氧化钇（Y₂O₃）掺杂氧化锆的立方相（c-ZrO₂）是等电子的，因为两个Zr(4+)离子被两个Y(3+)离子取代，再加上一个电荷补偿氧空位（Ovac）。典型的掺杂浓度包括3%、8%、10%和12摩尔%。对于这些浓度以及所有低于40摩尔%的浓度，在X射线或中子衍射实验中均未观察到具有长程有序的相。因此，预测局部缺陷结构以及缺陷之间的相互作用备受关注。由于可能的缺陷拓扑结构数量非常大，要对所有这些结构进行可靠的总能量计算成本过高，所以到目前为止这是不可能实现的。先前的理论研究仅考虑了一些代表性结构。在本研究中，使用经典建模和密度泛函理论相结合的方法对低能量缺陷结构进行全面搜索，以确定在稀释极限3.2摩尔%下的低能量孤立缺陷结构。通过分析使用最佳可用的Born-Mayer-Huggins经验势模型、点电荷模型、DFT计算的能量以及在简谐近似中估计的局部应变能，讨论了与低能量DFT结构相关的主要化学和物理描述符。研究发现，经验势模型再现了一系列局部应变弛豫结构中DFT能量增加的一般趋势，但在预测一些不正确的低能量结构以及发现一些亚稳结构不稳定方面都不可靠。发现更好的低能量缺陷结构预测器是在缺陷的未弛豫几何结构处计算的简单点电荷模型的总静电能。此外，在简谐近似中有效地估计了未掺杂c-ZrO₂的虚声子模式的应变弛豫能，但发现其在确定低能量缺陷结构方面并不重要。这些结果使我们能够提出一组易于计算的描述符，可用于识别低能量YSZ缺陷结构，消除了需要考虑的组合复杂性和缺陷结构数量。
