Novel Vibrational Modes and Thermal Properties in Systems with Large, Open Unit Cells

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This individual investigator award is to a senior professor at the University of California in Santa Cruz. The project involves a series of local structure (XAFS) measurements on systems whichhave large, open unit cells. Such materials have a rich array of vibrational motions, many of which are not observed in simple structures. In particular, there can be a large number of low energy optical phonon modes, some of which involve rotations of nearly rigid atomic. Such rigid unit vibrational modes have been proposed explain the novel negative thermal expansion coefficient of ZrW2O8 and other open structures. Another set of open structure materials are the partially filled skutterudite compounds. Here rare earth atoms can be incorporated into a void in the unit cell and strongly suppress the thermal conductivity. The XAFS measurements will clarify whether the proposed structural models are correct and provide a basis for developing better thermoelectric compounds and low thermal expansion materials. The work will provide educational opportunities for both graduate and undergraduate students to work as a team under the PI. Students will gain experience working at a national facility as well as learn experimental design and data analysis at the home institution, UCSC. %%%A number of complex materials, containing open spaces within the crystal structure, exhibit unusual thermal properties. They are potentially important for a variety of practical applications, including better thermoelectric coolers and materials that do not shrink when cooled.The voids in such materials allow the atoms to vibrate in new ways and many of the unusual and enhanced thermal properties are attributed to such motions. In one set of materials, clusters of atoms are thought to vibrate nearly as a single unit; in others it has been proposed thateach added impurity atom "rattles around" inside a void in the structure. This is an individual investigator award to a senior professor at the University of California at Santa Cruz to fund the detailed experimental measurements of these types of motion crucial for developing an understanding of these unusual thermal properties. The work will provide educational opportunities for both graduate and undergraduate students to work as a team under the PI. Students will gain experience working at a national facility as well as learn experimental design and data analysis at the home institution, UCSC. ***