NSF/DMR-BSF: The Effects of Configurational Disorder on Polaron Transport

非技术摘要：氧化物是众所周知的低电子电导率材料。但仔细研究后发现，它们传导电荷的能力存在很大的差异，其中大部分无法解释。例如，在钴锰氧化物中，电导率可以增加多达10倍，这取决于钴与锰的比例。当只有两种成分时，现有的理论是准确的，例如，只有钴和氧，但当包括第三种成分时，理论就崩溃了。在这个项目中，该团队正在研究氧化物的一个基本问题：原子无序如何影响氧化物的导电性。通过阐明这些机制，该团队正在开辟新的途径来定制氧化物的导电性，这对于依赖氧化物绝缘特性的应用（如半导体晶体管）或可能受益于导电性增加的应用（如电池和燃料电池）非常重要。为了向公众传播和推广科学，该团队正在创建一个网站，提供在线培训视频，以加强国际科学推广和美国-以色列合作伙伴关系，将科学用于和平目的，该团队正在为借阅图书馆实验设计一个关于氧化物电子导电性的模块。该套件符合特定的下一代科学标准，并使用探索性的动手活动，以帮助体验式学习。技术摘要：这项工作的目的是调查三元氧化物的一个基本问题：当有一个以上类型的阳离子时，电荷载流子如何通过尖晶石晶格移动。众所周知，氧化物中电荷输运的最主要机制是通过遵循极化子模型的跳跃。但是极化子跳跃模型太粗糙，无法解释三元过渡金属尖晶石中发生的构型差异，其中两种不同的阳离子类型可以在两种阳离子晶格位置之间表现出很大程度的无序，并具有多种氧化态。因此，阳离子无序和阳离子氧化态对电子电导率的影响还没有很好地描述在多元尖晶石。为了研究这个问题，研究小组正在合成三元氧化物尖晶石，表征阳离子无序，并将无序与电子输运联系起来。该团队正在使用X射线发射光谱，高角度环形暗场成像和电子能量损失光谱来确定占位，氧化态和局部原子分离。实验结果与理论相结合，以了解机制，并概述了一个全球模型的运输。该团队正在使用具有周期性和非周期性边界条件的DFT类型计算，并正在分析与氧化物材料中的跳跃机制相关的电荷传输特性，以确定电荷载流子是否位于特定阳离子分布和浓度的离域（重叠）或局域态。理解极化子的基本机制对于包括分子结构和电能存储在内的领域来说是一个巨大的挑战，在这些领域中，不良电子导体材料中电荷传输的细节可能会产生很大的影响。理解阳离子构型无序和极化子跳跃之间的关系可以导致具有定制性质的设计者氧化物。该小组正在通过创建一个网站向公众推广科学，该网站提供在线培训视频，用于国际科学推广，并加强美国和以色列利用科学用于和平目的的伙伴关系，该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的评估被认为值得支持。影响审查标准。

Nontechnical Abstract: Oxides are well known as materials with low electronic conductivity. But upon closer examination, there is a great deal of variability in their ability to conduct charge, and much of this is unexplained. For instance, in cobalt manganese oxides the conductivity can increase up to 10 times depending on the cobalt to manganese ratio. The existing theories are accurate when there are only two components, for instance, just cobalt and oxygen, but when a third component is included, the theories break down. In this project, the team is investigating a fundamental question of oxides: how does atomic disorder affect conductivity in oxides. By elucidating these mechanisms, the team is opening up new avenues to tailor the conductivity in oxides, important for applications that rely on the insulating properties of oxides, as in semiconductor transistors, or for applications that could benefit from increased conductivity, as in batteries and fuel cells. To disseminate and promote science to the public the team is creating a website with on-line training videos to strengthen international science outreach and the US-Israel partnership using science for peaceful purposes, and the team is designing a module on electronic conductivity in oxides for Lending Library Experiments. The kit aligns with specific Next Generation Science Standards and uses exploratory hands-on activities to help experiential learning.Technical Abstract: The purpose of this work is to investigate a fundamental question in ternary oxides: how do charge carriers move through a spinel crystal lattice when there is more than one type of cation. It is known that the most prominent mechanism of charge transport in oxides is through hopping that follows the polaron models. But the polaron hopping models are too crude to account for the configurational differences that occur in ternary transition metal spinels, where the two different cation types can exhibit large degrees of disorder between the two cation lattice sites and possess a variety of oxidation states. As a consequence, the effects of cation disorder and cation oxidation states on electronic conductivity has not been well-described in multinary spinels. To investigate this question the team is synthesizing ternary oxide spinels, characterizing the cation disorder, and correlating the disorder with the electronic transport. The team is using x-ray emission spectroscopy, high-angle annular dark-field imaging, and electron energy loss spectroscopy to determine site occupancy, oxidation states, and local atomic segregation. The experimental results are coupled with theory to understand the mechanisms and outline a global model for transport. The team is using DFT-type calculations with periodic and non-periodic boundary conditions, and is analyzing charge transport characteristics related to hopping mechanisms in oxide materials to determine if the charge carriers lie on delocalized (overlapping) or localized states for specific cation distributions and concentrations. Understanding the fundamental mechanisms of polarons is a grand challenge for fields including molecular architectures, and electrical energy storage, where details of charge transport in poor electron-conductor materials could have a large impact. Understanding the relationship between cation configurational disorder and polaron hopping could lead to designer oxides with tailored properties. The team is promoting science to the public by creating a website with on-line training videos for international science outreach and to strengthen the US-Israel partnership using science for peaceful purposes, and designing an experiment on electronic conductivity in oxides for Lending Library Experiments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.