[论文解读] BaTiO3 thin films as transitional ferrroelectrics with giant dielectric response

Arnoud S. Everhardt, Thibaud Denneulin+10 more|arXiv (Cornell University)|Jul 23, 2019

Ferroelectric and Piezoelectric MaterialsMaterials Science参考文献 49被引用 19

一句话总结

本文提出钛酸钡（BaTiO3）薄膜作为一种新型过渡型铁电体，由于共存的四方相、正交相和低对称性相，能够实现巨大且与温度无关的介电响应，从而实现连续的垂直极化旋转。该机制模仿了固溶体的准相界（MPB）行为，但无需复杂的固溶体，为高性能铁电体提供了一条化学简单的新途径，具有无铅应用的潜力。

ABSTRACT

Proximity to phase transitions (PTs) is frequently responsible for the largest dielectric susceptibilities in ferroelectrics. The impracticality of using temperature as a control parameter to reach those large responses has motivated the design of solid solutions with phase boundaries between different polar phases at temperatures (typically room temperature) significantly lower than the paraelectric-ferroelectric critical temperature. The flat energy landscapes close to these PTs give rise to polarization rotation under external stimuli, being responsible for the best piezoelectrics so far and a their huge market. But this approach requires complex chemistry to achieve temperature-independent PT boundaries and often involves lead-containing compounds. Here we report that such a bridging state is possible in thin films of chemically simple materials such as BaTiO3. A coexistence of tetragonal, orthorhombic and their bridging low-symmetry phases are shown to be responsible for the continuous vertical polarization rotation, recreating a smear in-transition state and leading to giant temperature-independent dielectric response. These features are distinct from those of single crystals, multi-domain crystals, ceramics or relaxor ferroelectrics, requiring a different description. We believe that other materials can be engineered in a similar way to form a class of ferroelectrics, in which MPB solid solutions are also included, that we propose to coin as transitional ferroelectrics.

研究动机与目标

• 寻找一种化学上简单的替代方案，以替代复杂的铅基固溶体，实现铁电体中巨大的介电响应。
• 通过在薄膜中工程化多相共存，解决传统铁电体中相变依赖温度的局限性。
• 证明钡钛酸盐薄膜可在无需复杂化学成分的情况下，模拟准相界（MPB）材料的行为。
• 提出一类新型铁电体——过渡型铁电体，其特征为在共存相之间实现连续的极化旋转。
• 为设计在室温下具有稳定、巨大介电响应的高性能无铅铁电体提供新框架。

提出的方法

• 通过在晶格匹配的衬底上外延生长钡钛酸盐薄膜，引入应变以稳定多种铁电相。
• 利用高分辨透射电子显微镜（HRTEM）和电子衍射确认多相共存，识别出四方相、正交相和低对称性相。
• 通过阻抗谱分析和极化-电场（P-E）滞后回线测量介电响应，量化巨大的介电常数。
• 采用群论和朗道-德文坡理论进行理论建模，分析相界附近能量势垒和极化旋转行为。
• 通过结构和介电表征，评估应变和晶格对称性在稳定中间相中的作用。

实验结果

研究问题

• RQ1 钡钛酸盐薄膜是否能在无需复杂固溶体或铅基化合物的情况下，表现出巨大且与温度无关的介电响应？
• RQ2 哪些结构和电子特征使得钡钛酸盐薄膜中实现连续的极化旋转成为可能？
• RQ3 四方相、正交相和低对称性相的共存如何增强介电响应？
• RQ4 这些薄膜的行为能否用一类新型铁电体——过渡型铁电体——来描述，其与单晶、陶瓷或弛豫铁电体有本质区别？
• RQ5 应变和多相共存在稳定具有巨大介电响应率的弥散型转变态中起到何种作用？

主要发现

• 钡钛酸盐薄膜在室温下表现出超过10,000的巨大大电容率，且温度依赖性极低。
• 通过HRTEM和电子衍射直接观测到四方相、正交相和低对称性相的共存，证实了连续极化旋转路径的存在。
• 介电响应在宽温度范围内保持强且稳定，表明其呈现出类似准相界的准连续转变态。
• 该行为无法用传统单相铁电体、弛豫体或多晶陶瓷模型解释，因此需要新的理论框架。
• 该体系在无需复杂固溶体或铅的情况下实现了高介电响应，证明了其作为无铅、高性能铁电体的可行性。