Work

Anisotropic low-energy vibrational modes as an effect of cage geometry in the binary barium silicon clathrate Ba24Si100

Year: 2020

Type: article

Abstract: The low lattice thermal conductivity in inorganic clathrates has been shown recently to be related to the low-energy range of optical phonons dominated by motions of guest atoms trapped in a network of host covalent cages. A promising route to further reduce the heat conduction, and increase the material efficiency for thermoelectric heat waste conversion, is then to lower the energy of these guest-weighted optical phonons. In the present work, the effect of the host cage geometry is explored. The lattice dynamics of the binary type-IX clathrate, $\mathrm{B}{\mathrm{a}}_{24}\mathrm{S}{\mathrm{i}}_{100}$, has been investigated experimentally by means of inelastic neutron scattering as a function of temperature between 5 and 280 K, and computed by ab initio density functional techniques. It is compared with the lattice dynamics of $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{46}$, the simplest representative of the well-known type-I clathrate structure. The binary $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{46}$ and $\mathrm{B}{\mathrm{a}}_{24}\mathrm{S}{\mathrm{i}}_{100}$ materials have both a cubic unit cell made of different Si cages. The energies, the degree of anharmonicity as well as the anisotropy of the optical phonon modes weighted by Ba motions are found to depend strongly on the size and shape of the cages. The lowest optical phonon energies in $\mathrm{B}{\mathrm{a}}_{24}\mathrm{S}{\mathrm{i}}_{100}$ are found around 2.5--4 meV, while those in $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{46}$ have higher energies around 7--9 meV. The low-lying optical phonons in $\mathrm{B}{\mathrm{a}}_{24}\mathrm{S}{\mathrm{i}}_{100}$ are mainly weighted by the motion of Ba in the opened $\mathrm{S}{\mathrm{i}}_{20}$ cage, which does not exist in $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{46}$. Moreover, the Ba vibrations within the opened $\mathrm{S}{\mathrm{i}}_{20}$ cages are found intrinsically anisotropic, strongly dispersionless in some directions, and exhibit a significant anharmonicity, which is not observed for any optical phonon modes in $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{46}$.

Source: Physical review. B./Physical review. B

DOI: 10.1103/physrevb.101.224302

Authors: Romain Viennois, Michael Marek Koza, Régis Debord, Pierre Toulemonde, Hannu Mutka, Stéphane Pailhès