氧化铈电空间取代化学的原子尺度研究
Atomic-scale insights into electro-steric substitutional chemistry of cerium oxide
摘要：
氧化铈（CeO 2）是一种最有前途的离子和电子混合导电材料。以前的原子论分析广泛地涵盖了替代对氧空位迁移的影响。然而，深入分析阳离子取代三价阳离子以外的作用很少被探索。在这里，我们调查可溶性一价，二价，三价和四价阳离子取代基。通过结合经典模拟和量子力学计算，我们提供了一个洞察取代基阳离子和氧空位之间的缺陷缔合能，以及它们对扩散机制的影响。模拟结果表明，低价阳离子取代体系的氧离子扩散系数为Gd>Ca>Na。对于相同的电荷，与Ce阳离子的较大尺寸失配产生较低的氧离子扩散率，即，Na>K，Ca>Ni，Gd> Al。基于这些趋势，我们确定了可以调节氧离子扩散率的物种：我们估计，在800 K下，对于GdxCe 1-xO 2-x/2，CaxCe 1-xO 2-x和NaxCe 1-xO 2 -3x/2，实现快速氧离子输运的最佳氧空位浓度为2.5%。值得注意的是，这样的浓度不是恒定的，并且随着温度的升高而逐渐向更高的值移动。我们发现，共同取代可以增强单取代的影响超出预期的简单添加。此外，我们确定优先的氧离子迁移途径，这说明了取代基阳离子在确定氧离子迁移的能量障碍的电空间效应。对控制氧扩散系数和迁移能量的因素的这种基本见解将使得能够定义用于调节材料的离子性质的设计标准，例如，通过共掺杂。
英文摘要：
Cerium oxide (ceria, CeO2) is one of the most promising mixed ionic and electronic conducting materials. Previous atomistic analysis has covered widely the effects of substitution on oxygen vacancy migration. However, an in-depth analysis of the role of cation substitution beyond trivalent cations has rarely been explored. Here, we investigate soluble monovalent, divalent, trivalent and tetravalent cation substituents. By combining classical simulations and quantum mechanical calculations, we provide an insight into defect association energies between substituent cations and oxygen vacancies as well as their effects on the diffusion mechanisms. Our simulations indicate that oxygen ionic diffusivity of subvalent cation-substituted systems follows the order Gd>Ca>Na. With the same charge, a larger size mismatch with Ce cation yields a lower oxygen ionic diffusivity, i.e., Na>K, Ca>Ni, Gd>Al. Based on these trends, we identify species that could tune the oxygen ionic diffusivity: we estimate that the optimum oxygen vacancy concentration for achieving fast oxygen ionic transport is 2.5% for GdxCe1-xO2-x/2, CaxCe1-xO2-x and NaxCe1-xO2-3x/2 at 800 K. Remarkably, such a concentration is not constant and shifts gradually to higher values as the temperature is increased. We find that co-substitutions can enhance the impact of the single substitutions beyond that expected by their simple addition. Furthermore, we identify preferential oxygen ion migration pathways, which illustrate the electro-steric effects of substituent cations in determining the energy barrier of oxygen ion migration. Such fundamental insights into the factors that govern the oxygen diffusion coefficient and migration energy would enable design criteria to be defined for tuning the ionic properties of the material, e.g., by co-doping.
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