[论文解读] Selective control of oxygen sublattice stability by epitaxial strain in Ruddlesden-Popper films

Tricia L. Meyer, Lu Jiang+9 more|arXiv (Cornell University)|Aug 27, 2015

Electronic and Structural Properties of Oxides参考文献 52被引用 15

一句话总结

本研究证明，外延张力应变选择性地稳定了钙钛矿型钙锶铜氧化物（LSCO）薄膜中的氧空位，导致空位形成吉布斯自由能降低，并优先占据赤道氧位点。与传统的晶格膨胀相反，这种效应引起晶格收缩，源于局域结构响应，为复杂氧化物中应变调控的氧非化学计量提供了新见解。

ABSTRACT

Oxygen-defect control has long been considered an influential tuning knob for producing various property responses in complex oxide films. In addition to physical property changes, modification to the lattice structure, specifically lattice expansion, with increasing oxygen vacancy concentrations has been reported often and has become the convention for oxide materials. However, the current understanding of the lattice behavior in oxygen-deficient films becomes disputable when considering compounds containing different bonding environments or atomic layering. Moreover, tensile strain has recently been discovered to stabilize oxygen vacancies in epitaxial films, which further complicates the interpretation of lattice behavior resulting from their appearance. Here, we report on the selective strain control of oxygen vacancy formation and resulting lattice responses in the layered, Ruddlesden-Popper phases, La1.85Sr0.15CuO4. We found that a drastically reduced Gibbs free energy for oxygen vacancy formation near the typical growth temperature for tensile-strained epitaxial LSCO accounts for the large oxygen non-stoichiometry. Additionally, oxygen vacancies form preferentially in the equatorial position of the CuO2 plane, leading to a lattice contraction, rather than the expected expansion, observed with apical oxygen vacancies. Since oxygen stoichiometry plays a key role in determining the physical properties of many complex oxides, the strong strain coupling of oxygen nonstoichiometry and the unusual structural response reported here can provide new perspectives and understanding to the structure and property relationships of many other functional oxide materials.

研究动机与目标

• 理解外延应变如何影响复杂氧化物薄膜中氧空位的形成。
• 研究钙钛矿型钙锶铜氧化物在应变下对氧非化学计量的结构响应。
• 解决在具有不同键合环境的层状氧化物中，氧空位形成时晶格行为的矛盾现象。
• 建立通过应变工程选择性控制氧亚晶格稳定性的机制。
• 为功能氧化物材料的结构-性能关系提供新见解。

提出的方法

• 在诱导张力应变的基底上外延生长了La1.85Sr0.15CuO4（LSCO）薄膜。
• 利用X射线衍射和共振软X射线散射技术探测晶格参数和氧空位分布。
• 采用第一性原理计算评估应变下氧空位形成的吉布斯自由能。
• 利用光谱学和散射技术分析特定位点的氧空位局域化。
• 通过能量最小化模型评估氧亚晶格在应变依赖下的热力学稳定性。
• 对轴向与赤道氧空位对晶格体积的影响进行了对比分析。

实验结果

研究问题

• RQ1 外延张力应变如何影响钙钛矿型氧化物中氧空位的热力学稳定性？
• RQ2 为何在应变LSCO薄膜中，氧空位形成导致晶格收缩而非膨胀？
• RQ3 在张力应变下，哪种氧位点（赤道或轴向）更倾向于容纳空位？
• RQ4 钙钛矿型钙钛矿相的层状结构如何改变氧非化学计量对晶格响应的常规行为？
• RQ5 外延应变在多大程度上可用于选择性控制氧亚晶格的稳定性？

主要发现

• 张力应变显著降低了LSCO薄膜中氧空位形成的吉布斯自由能，使得在典型生长温度下可实现较大的氧非化学计量。
• 氧空位优先在CuO2平面内的赤道氧位点形成，而非轴向位点。
• 在赤道位点形成空位会引发晶格收缩，与传统氧化物中预期的膨胀行为相反。
• 晶格响应表现出强烈的各向异性，收缩主要沿c轴发生，源于局域结构弛豫。
• 所观察到的行为源于应变对特定氧亚晶格的稳定化作用，使空位形成与整体晶格膨胀解耦。
• 这种选择性控制机制为精确调控复杂氧化物中的氧非化学计量和功能特性提供了新途径。