An AB initio study of the guest atom rattling in thermoelectric Sn clathrates
Date
2018
Authors
Egbele, Peter Odion
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Abstract
Semiconducting compounds with a clathrate hydrate-type I crystal structure are of growing interest as potential thermoelectric materials. The need to maximize the potential of these materials has informed increasing research to gain understanding on what is responsible for the low thermal conductivity in these compounds. The lattice thermal conductivity of binary type-I Sn clathrates, Cs8Sn4422, Rb8Sn4422
and K8Sn4422 was investigated using ab initio calculations based on density functional theory DFT. It is con rmed that vacancy formation in the stoichiometric compounds stabilizes the structure. In all the compounds the guest atoms interact with the framework and cannot be treated as weakly acting `rattlers'. Using a ctitious Sn46 framework, it was shown that in the binary compounds the acoustic modes are suppressed to lower frequencies. This leads to a slight (35%) reduction of phonon group velocities in the acoustic modes compared to the framework. However the major cause of the reduction
of the lattice thermal conductivities compared to a Sn46 framework, can be traced to the increase in the phonon-phonon scattering interactions which reduce the phonon lifetimes drastically from (175 ps to 25 ps). The phonon life times decrease over the whole frequency range and the relative life times decrease from Cs8Sn4422 to Rb8Sn4422 to K8Sn4422. The relative thermal conductivities scale in the reverse order.
Despite the order of the values of the relative thermal conductivities, Cs8Sn4422 is predicted to have the largest gure of merit, greater than 2 at room temperature.
Description
Thesis submitted to the Faculty of Science
University of the Witwatersrand,
in ful llment of the requirements for
the degree of Doctor of Philosophy (PhD)
School of physics,
University of the Witwatersrand,
Johannesburg.
2018
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Citation
Egbele, Peter Odion (2018) An AB initio study of the guest atom rattling in thermoelectric Sn clathrates, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/27217>