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

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, Ba24Si100, has been investigated experimentally by means of inelastic neutron scattering as a function of temperature between 5 K and 280 K, and computed by ab-initio density functional techniques. It is compared with the lattice dynamics of Ba8Si46, the simplest representative of the well-known type-I clathrate structure. The binary Ba8Si46 and Ba24Si100 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 2 the size and shape of the cages. The lowest optical phonon energies in Ba24Si100 are found around 2.5-4 meV, while those in Ba8Si46 have higher energies around 7-9 meV. The low-lying optical phonons in Ba24Si100 are mainly weighted by the motion of Ba in the opened Si20 cage, which doesn't exist in Ba8Si46. Moreover, the Ba vibrations within the opened Si20 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 Ba8Si46.