dc.description.abstract | GeTe is a chalcogenide with semi-metallic conductivity and ferroelectricity. The ferroelectric temperature is 670 K. Because it has higher electrical conductivity and higher thermal conductivity than other IV-VI compounds (8W/m -1K-1), it’s suitable as a thermoelectric material. However, there are a large number of Ge vacancies in GeTe which decreases the seebeck coefficient and affects the ZT value. Therefore, if the holes concentration can be reduced through doping, its thermoelectric performance can be improved. It is known from other studies that the doping of Sb and Bi in GeTe will affect electrons and phonons and improve the thermoelectricity value. Therefore, in this experiment, we will focus on discussing the influence of impurities on phonon scattering and the reduction of thermal conductivity after sample doping.
We used cold neutron triple-axis spectrometer SIKA in Ansto for inelastic neutron scattering measurement. First, we measured the elastic neutron scattering pattern in the temperature range of 300 K to 710 K. It was found that the sample changed from R3m to Fm3̅m phase in the temperature range of 625 K to 710 K, where two structural phases coexist. In addition, we draw the phonon dispersion diagram of Ge0.86Sb0.08Bi0.06Te in the temperature range of 300 K to 680 K, and obtain the Brillouin zone boundary lattice harmonic energy Eh, electron-phonon interaction energy Ee-p and anharmonic energy Ea. It is found that in this temperature range lattice anharmonicity and very strong electron-phonon interaction occur. In addition, due to the strong electron-phonon scattering, the phonon energy is softened. The properties of the phonon group velocity, phonon lifetime, and propagation length are obtained by fitting. It is known that the sample has a shorter phonon lifetime and phonon propagation length after mixing, and the group velocity will change to a negative value at a large q value. These factors limit the heat transfer and reduce the lattice thermal conductivity of Ge0.86Sb0.08Bi0.06Te, thereby increasing the ZT value of the sample. | en_US |