Study on the Lattice Dynamics of the Argyrodite Ag$_{8}$GeTe$_{6}$

Ag$_{8}$GeTe$_{6}$~was initially studied as a super ionic-electronic mixed conductor in the 1970s, and more recently has attracted new interest for its thermoelectric performance. A key to the desirable thermoelectric performance of Ag$_{8}$GeTe$_{6}$ is its exceptionally low lattice thermal conductivity ($\sim$ 0.25W/m*K at 300K), which is intimately related to its structure, consecutive structural instabilities, and unusual lattice dynamics (e.g., anharmonicity). In this work, we have studied Ag$_{8}$GeTe$_{6}$ by means of thermal conductivity, electrical conductivity, Seebeck coefficient, Hall coefficient, magnetic susceptibility, resonant ultrasound spectroscopy (RUS), photoacoustic spectroscopy, and synchrotron x-ray diffraction at low temperatures in order to further understand the coexistence of mixed conduction and high thermoelectric performance at elevated temperatures.