Chemical Engineering & Materials Science Department, Michigan State University, East Lansing, MI 48824, USA.
Phys Chem Chem Phys. 2018 Jun 6;20(22):15293-15299. doi: 10.1039/c8cp01219a.
Accurate characterization of chemical strain is required to study a broad range of chemical-mechanical coupling phenomena. One of the most studied mechano-chemically active oxides, nonstoichiometric ceria (CeO2-δ), has only been described by a scalar chemical strain assuming isotropic deformation. However, combined density functional theory (DFT) calculations and elastic dipole tensor theory reveal that both the short-range bond distortions surrounding an oxygen-vacancy and the long-range chemical strain are anisotropic in cubic CeO2-δ. The origin of this anisotropy is the charge disproportionation between the four cerium atoms around each oxygen-vacancy (two become Ce3+ and two become Ce4+) when a neutral oxygen-vacancy is formed. Around the oxygen-vacancy, six of the Ce3+-O bonds elongate, one of the Ce3+-O bond shorten, and all seven of the Ce4+-O bonds shorten. Further, the average and maximum chemical strain values obtained through tensor analysis successfully bound the various experimental data. Lastly, the anisotropic, oxygen-vacancy-elastic-dipole induced chemical strain is polarizable, which provides a physical model for the giant electrostriction recently discovered in doped and non-doped CeO2-δ. Together, this work highlights the need to consider anisotropic tensors when calculating the chemical strain induced by dilute point defects in all materials, regardless of their symmetry.
准确描述化学应变对于研究广泛的化学-机械耦合现象至关重要。氧化铈（CeO2-δ）是研究最多的机械化学活性氧化物之一，仅通过假设各向同性变形的标量化学应变来描述。然而，结合密度泛函理论（DFT）计算和弹性偶极张量理论表明，立方氧化铈（CeO2-δ）中的短程键畸变和长程化学应变都是各向异性的。这种各向异性的起源是当形成中性氧空位时，氧空位周围的四个铈原子（两个变为 Ce3+，两个变为 Ce4+）之间的电荷离域。在氧空位周围，六个 Ce3+-O 键伸长，一个 Ce3+-O 键缩短，并且所有七个 Ce4+-O 键缩短。此外，通过张量分析获得的平均和最大化学应变值成功地约束了各种实验数据。最后，各向异性、由氧空位-弹性偶极子引起的化学应变具有极化性，为最近在掺杂和未掺杂的 CeO2-δ 中发现的巨大电致伸缩提供了物理模型。总之，这项工作强调了在计算所有材料中稀释点缺陷引起的化学应变时需要考虑各向异性张量的必要性，无论其对称性如何。
