Solid State Chemistry of Chalcogenides for Materials Discovery

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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.