Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
Sci Rep. 2017 Jan 25;7:41301. doi: 10.1038/srep41301.
Vacancies play a pivotal role in affecting ferroelectric polarization and switching properties, and there is a possibility that ferroelectricity may be utterly eliminated when defects render the system metallic. However, sufficient quantitative understandings of the subject have been lacking for decades due to the fact that vacancies in ferroelectrics are often charged and polarization in charged systems is not translationally invariant. Here we perform first-principles studies to investigate the influence of vacancies on ferroelectric polarization and polarization switching in prototypical BaTiO of tetragonal symmetry. We demonstrate using the modern theory of polarization that, in contrast to common wisdom, defective BaTiO with a large concentration of vacancies (or , or ) possesses a strong nonzero electric polarization. Breaking of Ti-O bonds is found to have little effect on the magnitude of polarization, which is striking. Furthermore, a previously unrecognized microscopic mechanism, which is particularly important when vacancies are present, is proposed for polarization switching. The mechanism immediately reveals that (i) the switching barrier in the presence of is small with ΔE = 8.3 meV per bulk formula cell, and the polarization is thus switchable even when vacancies exist; (ii) The local environment of vacancy is surprisingly insignificant in polarization switching. These results provide profound new knowledge and will stimulate more theoretical and experimental interest on defect physics in FEs.
空位在影响铁电极化和开关性能方面起着至关重要的作用，当缺陷使系统金属化时，铁电性可能会完全消失。然而，由于铁电体中的空位通常带电，而带电系统中的极化不具有平移不变性，因此几十年来，人们对这一主题缺乏足够的定量理解。在这里，我们进行了第一性原理研究，以调查空位对四方对称原型钛酸钡中铁电极化和极化反转的影响。我们使用极化的现代理论证明，与普遍观点相反，具有大量空位（或 ，或 ）的缺陷钛酸钡具有很强的非零电场极化。我们发现，打破 Ti-O 键对极化强度几乎没有影响，这一点非常惊人。此外，我们提出了一个以前未被认识的微观机制，该机制在存在空位时尤为重要，用于极化反转。该机制立即表明：(i) 存在 时的开关势垒很小，ΔE=8.3meV/每个体相原胞，因此即使存在空位，极化也可以反转；(ii) 空位的局部环境在极化反转中并不重要。这些结果提供了深刻的新知识，并将激发更多关于铁电体中缺陷物理的理论和实验兴趣。
