Dr. Jifeng Sun from Prof. Singh's group and Dr. Jennifer L. Niedziela from Prof. Delaire's group will give research updates. Below are their presentation titles and abstracts.

Title: Recent results on n-type SrTiO3, resonant bonding in Ge-Sb-Te and Ta-As
Presenter: Jifeng Sun
Abstract:
We present investigation of three materials: La-doped n-type SrTiO3, amorphous and crystalline GST and the Ta-As system. For La-doped SrTiO3, we find that reasonable thermoelectric performance can be obtained at very high temperature. The special band structure of this material with degenerate t2g orbitals is important for this. For GST we elucidate a connection with the physics of ferroelectric materials that provides new insight into the so-called resonant bonding and the combination of high dielectric constant and low thermal conductivity that is favorable for a thermoelectric material. For Ta-As we discuss defects in relation to experiments showing a high density of planar defects in samples and robust Weyl physics in relation to these. We also present results for other Ta-As phases.

Title: Phonon contributions to lattice thermal conductivity in thermoelectric materials probed with neutron and x-ray scattering
Presenter: Jennifer L. Niedziela

Abstract:

Controlling lattice thermal conductivity in thermoelectric materials is critical to improving the performance of real materials for applications.  Probes of interatomic dynamics such as neutron and x-ray scattering are vital to unraveling the atomistic mechanisms contributing to lattice thermal conductivity.  In this talk, I will present the results of recent neutron and x-ray scattering studies on thermoelectric materials CuCrSe2, a triangular lattice chalcogenide, and natural mineral tetrahedrite.  CuCrSe2 exhibits a superionic transition at 360K. The effect of the Cu sublattice melting has a pronounced and reversible effect on the phonon behavior, resulting in a complete loss of coherence for phonon modes contributing to the optical band near 9 meV.  Through first principles simulations, we are able to ascertain that the phonon modes most strongly impacted by the transition are those confined to the x-y plane of the triangular Cu sublattice.  In natural mineral tetrahedrite, we observe a low energy phonon which disrupts the transverse acoustic phonons most responsible for thermal transport. This mode has an optical character, and is driven by incoherent oscillations of Cu atoms in a trigonal coordination with S atoms, a sublattice unit in the larger tetrahedrite structure. This mode displays strongly anharmonic behavior, showing a greater tendency to thermal transport disruption with increasing temperature.