Anharmonic lattice dynamics and thermal transport in type-I inorganic clathrates

Abstract

The anharmonic phonon properties of type-I filled inorganic clathrates Ba8Ga16Ge30 and Sr8Ga16Ge30 are obtained from the first-principles calculations by considering the temperature-dependent sampling of the potential energy surface and quartic phonon renormalization. Owing to the weak binding of guest atoms with the host lattice, the obtained guest modes undergo strong renormalization with temperature and become stiffer by up to 50% at room temperature in Sr8Ga16Ge30. The calculated phonon frequencies and associated thermal mean squared displace- ments are comparable with experiments despite the on-centering of guest atoms at cage centers in both clathrates. Lattice thermal conductivities are obtained in the temperature range of 50- 300 K accounting for three-phonon scattering processes and multi-channel thermal transport. The contribution of coherent transport channel is significant at room temperature (13% and 22% in Ba8Ga16Ge30 and Sr8Ga16Ge30) but is insufficient to explain the experimentally observed glass-like thermal transport in Sr8Ga16Ge30.

Single-channel or multi-channel thermal transport? Effect of higher-order anharmonic corrections on the predicted phonon thermal transport properties of semiconductors

“…Using the lowest-order transport theory, the predicted thermal conductivity of two compounds is lower than 0.2 W/m-K at a temperature of 300 K which is comparable to the reported lowest thermal conductivity of solids in literature [9,15,21]. For such low thermal conductivity materials, the phonon mean free paths could become shorter than the Ioffe-Regel limit, and in such cases, an additional contribution from wave-like coherent transport channel becomes significant towards the thermal transport [15,17,26].…”

“…2 are bare-harmonic force constant (as obtained from finitedifference or density functional perturbation theory and without renormalization). The phonon-phonon scattering rates at the lowest-order of theory are limited to three-phonon processes and are obtained as [9,18,20,21]:…”

“…Further details on these methods are available in Refs. [9,21]. The computational details and the convergence of predicted results with the choice of numerical parameters are reported in Secs.…”

High-throughput computational discovery of 40 ultralow thermal conductivity and 20 highly anisotropic crystalline materials

“…3, the thermal conductivity of considered compounds correlates strongly (negative correlation) with thermal MSD, and, in general, materials with small (large) MSD result in a high (low) thermal conductivity. The MSD of Cs atoms in SbCsK 2 and BiCsK 2 , of Rb atoms in SbRbK 2 and that of Te atoms in Y 2 TeS 2 are larger than 0.08 Å2 at 300 K. Such large MSD are also observed in cage-like compounds, such as clathrates [25], and indicate rattler-like motion of concerned atoms resulting in a scattering of heat-carrying acoustic phonons and hence low material thermal con- ductivity [10,[25][26][27]. This is indeed the case for SbCsK 2 , BiCsK 2 , and SbRbK 2 , and the predicted thermal conductivity of all three of these compounds is lower than 0.3 W/m-K. For Y 2 TeS 2 , however, the thermal conductivity is high at 7.1 W/m-K despite the large MSD of 0.13 Å2 .…”