Lattice dynamics of strong negative thermal expansion materials

University Of Connecticut, Storrs CT

Investigators

Abstract

Non-technical abstract: Thermal expansion is a property that all materials possess and most commonly occurs as a volume increase in response to a temperature increase. However, recent new materials discoveries have revealed instances of negative thermal expansion (NTE), a peculiar phenomenon of heat-induced volume contraction. NTE materials are not currently used in industrial contexts, but hold strong application potential in stabilizing telecommunication components, as elements of electronic devices benefiting from thermally controlled stresses, and the potential to form rigid composite structural materials with engineered thermal properties. This project uses light- and X-ray-based spectroscopy to study excitations driving the NTE effect in a representative set of materials with the goal of achieving an understanding capable of leading new material discovery efforts. Training of the next generation's high-tech workforce through both graduate and undergraduate participants performing experimental activities at Connecticut's flagship state university and national user facilities is an integrated component of the project. In addition, a novel curricular effort offers junior and senior science majors the opportunity to develop demonstrations suitable for presentation on a document camera platform in large lecture halls for use in introductory physics classes serving science and engineering freshmen and sophomore students. Educational development for students and by students, guided by departmental faculty and staff, is a central theme of the Doc-cam demos project. Technical abstract: The past 10 years has seen an accelerated rate of discovery of materials with robust and sizable negative thermal expansion (NTE). The goal is to advance our understanding of NTE associated with dynamics of tightly bound, stiff molecular units and local atomic arrangements allowing for strong nuclear lattice fluctuations in a direction transverse to a structural linkage defining the unit cell. Because thermal expansion of any sign is necessarily an excited state effect and involves anharmonic interactions, the basic science motivation surrounds the likely possibilities that strong quantum fluctuations, rotational tunneling, and many-body nonlinear localization play a role in this remarkable effect. Inelastic X-ray scattering, infrared and terahertz absorption spectroscopy, and thermal transport measurements will be performed in a select subset of recently discovered NTE materials whose structures have related features, including members possessing the important perovskite structure. Detailed study of Brillouin zone boundary and zone-center modes believed to contribute will be assessed and compared across materials classes. Educational activities involve training a new generation of scientists through a novel project permitting advanced science majors to exercise their creative potential by developing new document camera demonstrations for use in large lecture settings. This curricular effort flips the demo room in the spirit of peer instruction and other recent science teaching innovations.