Applied Physics Seminar

Abstract:

In materials where the anharmonic atomic potential surface is easily accessible by thermal energy, the instantaneous structure can be qualitatively different from the average structure determined by elastic scattering methods. Our group has demonstrated that Raman crystallography can give information about both the instantaneous structure and the anharmonic lattice dynamics in these materials, allowing us to make novel hypotheses about the effect of anharmonicity on the functionality of the material. Raman crystallography was developed to provide information on the crystal structure, orientation, and local atomic coordinations of a material by utilizing the symmetry properties of a crystal's Raman response to polarized light. It can be used, for instance, to determine the orientation of a Si thin film without the need to measure x-ray diffraction. However, its power extends beyond simple structural characterization because the Raman response, which occurs on the order of picoseconds or less, contains information about the instantaneous configuration of atoms on this time scale and the anharmonicity in the structural dynamics.

In this talk I will demonstrate how Raman crystallography has allowed us to understand the structural dynamics of two technologically-critical materials systems: fast ion conductors and semiconductors for optoelectronic devices. I will demonstrate (i) how anharmonic relaxational motion is coupled to ionic transport in the prototypical fast ion conductor -AgI, and (ii) how its broad-featured spectrum reveals a clear crystallographic symmetry in Raman crystallography measurements that allows us to hypothesize a full explanation of the Raman spectrum. I will also report progress in our group on understanding the anharmonic lattice dynamics of several halide perovskites, which are being intensively researched for their impressive photovoltaic efficiencies. In both cases, I will discuss what the implications of the observed anharmonicity may be for the functional properties of the material.