Year: 2019
Type: article
Abstract: Inorganic clathrates such as ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{x}{\mathrm{Ge}}_{46\ensuremath{-}x}$ and ${\mathrm{Ba}}_{8}{\mathrm{Al}}_{x}{\mathrm{Si}}_{46\ensuremath{-}x}$ commonly exhibit very low thermal conductivities. A quantitative computational description of this important property has proven difficult, in part due to the large unit cell, the role of disorder, and the fact that both electronic carriers and phonons contribute to transport. Here, we conduct a systematic analysis of the temperature and composition dependence of low-frequency modes associated with guest species in ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{x}{\mathrm{Ge}}_{46\ensuremath{-}x}$ and ${\mathrm{Ba}}_{8}{\mathrm{Al}}_{x}{\mathrm{Si}}_{46\ensuremath{-}x}$ (``rattler modes''), as well as thermal transport in stoichiometric ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$. To this end, we account for phonon-phonon interactions by means of temperature-dependent effective interatomic force constants, which we find to be crucial in order to achieve an accurate description of the lattice part of the thermal conductivity. While the analysis of the thermal conductivity is often largely focused on the rattler modes, here it is shown that at room temperatures modes with $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\gtrsim}10\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ account for 50% of lattice heat transport. Finally, the electronic contribution to the thermal conductivity is computed, which shows the Wiedemann-Franz law to be only approximately fulfilled. As a result, it is crucial to employ the correct prefactor when separating electronic and lattice contributions for experimental data.
Institution: Chalmers University of Technology
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
Award: G767667885
OpenAlex ID: W2810208788