Towards the Rational Design of Materials Exhibiting Colossal Metal-Insulator Transitions

很少有物质的物理性质能像导电性那样在不同的材料之间表现出如此大的差异。材料可以是绝缘体、半导体和金属，实际上，不同材料携带电子电荷的独特能力支撑着现代技术的大多数方面。一类相对罕见的材料包括固体，它们在施加的外部刺激（如温度升高、电压施加、光源照明或受到拉伸应变）下突然从绝缘体转变为金属。更罕见的是，在接近室温的温度下，材料的性质会发生如此剧烈的变化。在材料研究部固态和材料化学项目的支持下，该项目专注于破译原子结构和化学键在这些突变中的变化，以期合理设计材料，可以在响应外部刺激时诱导其电导率转换多个数量级。这种活性利用了过渡金属氧化物结构的多功能性，过渡金属氧化物是一类化学成分和原子结构的精确细节可以被很好地控制的材料。该项目预计将极大地扩大现有材料的范围，这些材料在接近室温的情况下表现出与技术相关的特性可切换性，同时也解决了与此类固体中电子之间相互作用相关的基础科学问题。首席研究员专注于吸引从社区大学转学的学生在他们大学生涯的早期进行研究，以更好地利用这个经常被忽视的人才库。最后，该项目正在开发鼓舞人心的材料，为小学生提供纳米科学技术可能性的味道。该项目关注的是金属-绝缘体转变的有趣现象，在这种现象中，材料的导电性会随着温度、压力、电压、光激发或化学成分的变化而突然从绝缘转变为金属行为。电导率的这种突然的不连续变化是由材料中的局部价电子向流动电子的转变引起的，并且通常是由电子-电子相互作用支撑的。理解和调整电子相关仍然是物理科学的重大挑战之一。这项研究活动的基本前提是，在复杂的过渡金属氧化物中，原子间距离、电荷序基序和掺杂比可以通过改变插层阳离子的大小、电荷、化学计量和极化率来精确调节。活动范围包括复杂氧化物的化学合成与精确控制组成和结构，结构和组成特征与金属绝缘体过渡的相关性，应用衍射和光谱工具来探测这些过渡的机制基础，以及通过定义插层梯度来调制过渡。一项暑期研究活动将吸引从社区大学转到德州农工大学的本科生。将在公共科学活动中开发和传播以纳米科学为重点、旨在激励小学生的图画小说。

Non-Technical SummaryThere are few physical properties of matter that show as much variation across different materials as electrical conductivity. Materials can be insulators, semiconductors, and metals, and indeed the distinctive abilities of different materials to carry electronic charge underpin most aspects of modern technology. A relatively rare class of materials comprises solids that abruptly switch from being insulators to metals in response to an applied external stimulus such as an increase in temperature, application of a voltage, illumination with a light source, or being subjected to a tensile strain. Rarer still are materials that undergo such dramatic switching of properties close to room temperature. With support from the Solid State and Materials Chemistry program in the Division of Materials Research, this project focuses on deciphering the changes in atomic structure and chemical bonding across these abrupt transformations with a view towards rationally designing materials that can be induced to switch their conductivity across many orders of magnitude in response to external stimuli. The activity takes advantage of the structural versatility of transition metal oxides, a class of materials where the chemical composition and the precise details of the atomic structure can be finely controlled. The project is anticipated to greatly expand the available repertoire of materials that show such technologically relevant switchability of properties close to room temperature while also addressing fundamental science related to the interaction between electrons in such solids. The principal investigator is focused on engaging students transferring from community colleges in research early on in their college careers to better tap into this often overlooked talent pool. Finally, the project is developing inspirational material to provide elementary school students a flavor of the technological possibilities of nanoscience.Technical SummaryThe project is focused on the intriguing phenomenon of metal-insulator transitions wherein the electrical conductivity of a material abruptly switches from insulating to metallic behavior in response to temperature, pressure, voltage, photo-excitation, or change in chemical composition. Such an abruptly discontinuous change in electrical conductivity arises from the transformation of localized valence electrons to itinerant electrons in the material, and is often underpinned by electron-electron interactions. Understanding and tuning electron correlation remains one of the grand challenges of the physical sciences. The fundamental premise of the research activity is the idea that inter-atomic distances, charge-ordering motifs, and dopant ratios can be precisely modulated in complex transition metal oxides through variation of the size, charge, stoichiometry, and polarizability of intercalating cations. The activity spans chemical synthesis of complex oxides with precise control of composition and structure, correlation of structural and compositional characteristics to metal-insulator transitions, the application of diffraction and spectroscopic tools to probe the mechanistic basis for these transitions, and modulation of the transitions by defining intercalation gradients. A summer research activity will engage undergraduates transferring to Texas A&M from community colleges. Graphical novels focused on nanoscience and designed to motivate elementary school students will be developed and disseminated at public science events.