Solid State Chemistry of Chalcogenides for Materials Discovery

本研究的主要目标是发现和表征新型金属硫系化合物，并开发和了解它们的结构、化学键和物理性质，该研究由固态和材料化学计划支持。该合成方案的一个重要问题是，我们是否能够在中温条件下指导盐通量中发生的基本反应化学，以抑制不良化合物的形成，并有利于新化合物的结晶。本项目采用碱金属多硫化物助熔剂合成，生产含硫化物缩合单元的材料。良好定义的构建块存在于通量反应中，通过调节通量组成和温度来指导它们的形成，从而控制刘易斯碱度和氧化还原电位。此外，盐通量可以维持可调的动态平衡，这对于合成新的金属硫族化合物材料是重要的。据推测，这些通量性质的操纵将允许控制合成路线，以各种新的结构。在这种化学中使用的金属主要是主族金属和稀土金属，在某些情况下，还包括过渡金属。硫族化合物类的新材料有望具有吸引人的化学和物理性质，如离子交换、半导体（根据结构和成分的不同，具有0.5-3.0 eV的宽范围能带间隙）、金属、相变和非线性光学性质（特别是在红外区域产生很强的二次谐波）。预计新材料的许多物理特性将对技术影响和应用的进一步发展具有重大潜力。新材料的合成和晶体生长越来越被认为是对物理科学产生重大影响的重要研究基础，因此，诸如提出的计划既相关又及时。在这个固态和材料化学资助的项目下，新的硫系材料预计具有有用的化学和物理性质，如离子交换、半导体（根据结构和成分，具有0.5-3.0 eV的宽范围能带间隙）、金属、相变和非线性光学性质（特别是在红外区域产生很强的二次谐波）。在这个项目中使用了各种各样的实验表征工具，包括单晶和粉末x射线晶体学，使用内部和同步辐射，固态光学，红外和拉曼光谱，扫描和透射电子显微镜，差热分析和扫描量热法，以及电导率和光学二次谐波产生的测量。预计新材料的许多物理特性将对技术影响和应用的进一步发展具有重大潜力。在基层，固态和材料化学社区认识到开发合理的材料发现策略的巨大挑战。该项目通过开发新的综合方法来帮助解决这一挑战。对于硫属化合物类，设置合理的、科学驱动的基础，以获取最大的科技效益。具体的重点是培养和教授固态和材料化学的研究生，他们理解开发新材料作为新技术驱动力的重要性。该项目为研究生和本科生提供了学习当代材料化学研究所需的研究调查技能的重要机会。学生也接触到广泛的物理性质表征工具。学生在新材料的合成和晶体生长方面的培训对我们国家在关键材料方面的竞争力有积极的影响，并解决了日益增长的国家需求。学生也受益于高影响力的跨学科合作。最后，通过出版物广泛传播科学成果和知识将增进科学理解，并有望在其他地方激发进一步的研究活动。

TECHNICAL SUMMARYThe primary goals of this research supported by the Solid State and Materials Chemistry program are to discover and characterize new types of metal chalcogenide compounds and to develop and understand their structures, chemical bonding and physical properties. An important question of this synthesis program is whether we can guide the fundamental reaction chemistry occuring in salt fluxes at intermediate temperatures in order to suppress the formation of undesirable compounds and favor the crystallization of new ones. The project employs alkali metal polychalcogenide flux syntheses to afford materials containing condensed chalcogenide units. Well-defined building blocks are present in the flux reactions and their formation is guided by tuning the flux composition and temperature, which controls Lewis basicity and redox potential. In addition, the salt fluxes can sustain tunable dynamic equilibria that are important for the synthesis to be directed towards new metal chalcogenide materials. It is hypothesized that manipulation of these flux properties will allow the control of the synthetic routes toward a variety of new structures. The metals employed in this chemistry are primarily main group and rare earth metals and in select cases, transition metals. New materials of the chalcogenide class are expected with attractive chemical and physical properties such as ion-exchange, semiconductor (with a wide range of energy band gaps from 0.5-3.0 eV depending on structure and composition), metallic, phase-change and nonlinear optical properties (particularly very strong second harmonic generation in the infrared region). It is anticipated that many of the physical properties of the new materials will have significant potential for technological impact and further development in applications. NON-TECHNICAL SUMMARYSynthesis and crystal growth of new materials is increasingly recognized as an important underpinning of research that strongly impacts the physical sciences and thus programs such as the proposed one are both relevant and timely. Under this Solid State and Materials Chemistry funded program new chalcogenide materials are anticipated with useful chemical and physical properties such as ion-exchange, semiconductor (with a wide range of energy band gaps from 0.5-3.0 eV depending on structure and composition), metallic, phase-change and nonlinear optical properties (particularly very strong second harmonic generation in the infrared region). A wide variety of experimental characterization tools are employed in this project including single crystal and powder X-ray crystallography using in-house and synchrotron radiation, solid state optical, infrared and Raman spectroscopy, scanning and transmission electron microscopy, differential thermal analysis and scanning calorimetry, and measurements of electrical conductivity as well as optical second harmonic generation. It is anticipated that many of the physical properties of the new materials will have significant potential for technological impact and further development in applications. At a grassroots level, the solid state and materials chemistry community recognizes the grand challenge of developing rational materials discovery strategies. This project helps address this challenge by developing new synthesis methodologies. For the class of chalcogenides, a rational, science-driven foundation is set to extract maximum scientific and technological benefit. The specific focus is on training and teaching graduate students in solid state and materials chemistry who understand the importance of developing new materials as drivers for new technologies. The project provides important opportunities for graduate and undergraduate students to learn research investigative skills that are needed for contemporary materials chemistry research. The students are also exposed to a broad battery of physical property characterization tools. Student training in the synthesis and crystal growth of novel materials has a positive impact on our national competitiveness in key materials and addresses a growing national need. Students also benefit from high impact interdisciplinary collaborations. Finally, the broad dissemination of scientific results and knowledge through publication will enhance scientific understanding and hopefully stimulate further research activity elsewhere.