Avoided crossing of rattler modes in thermoelectric materials

Abstract

Engineering of materials with specific physical properties has recently focused on the effect of nano-sized 'guest domains' in a 'host matrix' that enable tuning of electrical, mechanical, photo-optical or thermal properties. A low thermal conductivity is a prerequisite for obtaining effective thermoelectric materials, and the challenge is to limit the conduction of heat by phonons, without simultaneously reducing the charge transport. This is named the 'phonon glass-electron crystal' concept and may be realized in host-guest systems. The guest entities are believed to have independent oscillations, so-called rattler modes, which scatter the acoustic phonons and reduce the thermal conductivity. We have investigated the phonon dispersion relation in the phonon glass-electron crystal material Ba(8)Ga(16)Ge(30) using neutron triple-axis spectroscopy. The results disclose unambiguously the theoretically predicted avoided crossing of the rattler modes and the acoustic-phonon branches. The observed phonon lifetimes are longer than expected, and a new explanation for the low kappa(L) is provided.

Twisting phonons in complex crystals with quasi-one-dimensional substructures

“…Indicating a similar symmetry in the displacement patterns of two hybridized modes (Supplementary Discussion), avoided crossings of this kind have been observed between the rattler mode and acoustic modes in the clathrate b-Ba 8 Ga 16 Ge 30 (ref. 35), where the on-centre rattler was found to flatten the phonon branches instead of providing a strong phonon scattering mechanism. Theoretical analysis has further suggested that an avoided crossing with a low-lying optical polarization results in a wide, flat transverse acoustic branch in b-Ba 8 Ga 16 Sn 30 with off-centre rattlers, and that such flattening of the acoustic phonon dispersion results in a glass-like plateau in the thermal conductivity 36,37 .…”

Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag8SnSe6

“…Crystalline solids with a low sound velocity should ideally behave as “phonon‐glass electron‐crystal”20 thermoelectric materials. Many of the recent efforts have been taken to seek for new thermoelectrics of intrinsic ultra‐low thermal lattice thermal conductivity, which were usually ascribed to the complex crystal structure,21 large molecular weight,22, 23 or lattice distortion 24. The essentiality for the minimal lattice thermal conductivity and therefore the high thermoelectric performance, is partially due to the low sound velocity as well, resulting from its weak chemical bonds and heavy atomic mass of constituent elements 25.…”

Heavy Doping by Bromine to Improve the Thermoelectric Properties of n‐type Polycrystalline SnSe

“…)3, 4, 5 and decreased thermal conductivity by defect engineering,6 and nanoengineering 7, 8, 9. An alternative way to achieve high ZT values is to seek new classes of TE materials with intrinsically low thermal conductivity, such as clathrates, skutterudites, sulfur‐based compounds, Ag 9 GaSe 6 , and SnSe etc 10, 11, 12, 13, 14…”