Thermal properties of germanium telluride close to the ferroelectric phase transition

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Dangic, Dorde
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University College Cork
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Improving thermoelectric efficiency is one of the most challenging problems in materials science. Recent work made a connection between increased phonon anharmonicity in the vicinity of the structural phase transition and the reduction of the lattice thermal conductivity, which could increase the thermoelectric performance of materials near structural phase transitions. In this thesis, we investigate the influence of the phase transition on the material properties in germanium telluride, an excellent thermoelectric material that undergoes a structural phase transition at around 700 K. We find that the enhanced acoustic - soft optical mode coupling causes negative thermal expansion close to the structural phase transition. Unexpectedly, the negative thermal expansion boosts phonon group velocities in this temperature region, countering the increased phonon anharmonicity and leading to an increase in the lattice thermal conductivity near the structural phase transition. The increased phonon anharmonicity in the vicinity of the phase transition causes non-Lorentzian shapes of the phonon spectral functions, questioning the traditional Boltzmann transport approach for the calculation of the lattice thermal conductivity. To address this issue, we implement a novel method of calculating lattice thermal conductivity, combining the Green-Kubo approach and the lattice (phonon) dynamics. We find that the Boltzmann transport equation underestimates the lattice thermal conductivity close to the phase transition. We perform molecular dynamics simulations of GeTe at different temperatures to confirm these findings. The phase transition in GeTe has a ferroelectric character as well. The formation of ferroelectric domains in GeTe has been observed experimentally. Here we also investigate the influence of ferroelectric domain walls on the thermoelectric properties of GeTe. We recognize several different types of domain walls that can occur in the. We calculate the structural, electronic, and transport properties of a few selected domain wall types. We find that domain walls offer a promising alternative for the reduction of lattice thermal conductivity in GeTe. Wider domain walls are shown to scatter phonons more efficiently, causing up to a 40% reduction of the lattice thermal conductivity. Additionally, we investigate the electronic transport properties of domain walls and find a two-fold enhancement of the Seebeck coefficient in the in-plane directions of domain walls. While significantly increasing the Seebeck coefficient, domain walls do not drastically suppress the electrical conductivity, leading to a significant increase of the power factor at charged domain walls in GeTe with respect to bulk (by a factor of 5). Finally, we propose a novel design of a nano-thermoelectric device that maximizes the beneficial transport properties of domain walls.
Phase transition , Thermoelectrics , Lattice thermal conductivity , Thermal expansion , Ferroelectric domain walls
Dangic, D. 2021. Thermal properties of germanium telluride close to the ferroelectric phase transition. PhD Thesis, University College Cork.