Part-crystalline part-liquid state and rattling-like thermal damping in materials with chemical-bond hierarchy

Abstract

Understanding thermal and phonon transport in solids has been of great importance in many disciplines such as thermoelectric materials, which usually requires an extremely low lattice thermal conductivity (LTC). By analyzing the finite-temperature structural and vibrational characteristics of typical thermoelectric compounds such as filled skutterudites and Cu 3 SbSe 3 , we demonstrate a concept of part-crystalline part-liquid state in the compounds with chemicalbond hierarchy, in which certain constituent species weakly bond to other part of the crystal. Such a material could intrinsically manifest the coexistence of rigid crystalline sublattices and other fluctuating noncrystalline sublattices with thermally induced largeamplitude vibrations and even flow of the group of species atoms, leading to atomic-level heterogeneity, mixed part-crystalline partliquid structure, and thus rattling-like thermal damping due to the collective soft-mode vibrations similar to the Boson peak in amorphous materials. The observed abnormal LTC close to the amorphous limit in these materials can only be described by an effective approach that approximately treats the rattling-like damping as a "resonant" phonon scattering.sublattice melting | partial Grüneisen parameter | first principles | anharmonicity U nderstanding thermal and phonon transport in solids has been of great importance in many disciplines such as thermoelectrics (1-3), phononic materials (4), and thermal management composites (5). The interplay among chemical bonds, lattice dynamics, and thermal transport in materials is also an attractive topic in condensed matter physics (6) and materials science (7). Thermal transport is a key issue in thermoelectric (TE) energy-conversion materials, which are regarded among the potential candidates for revolutionizing waste-heat recovery (2, 7-9). The dimensionless figure of merit of a TE material is defined as ZT = TS 2 σ=κ, where T, S, σ, and κ are the absolute temperature, Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. To improve the efficiency of TE conversion, many approaches aim at reducing the thermal conductivity, especially the lattice part, to a minimum level, namely the realization of phonon-glass-like thermal transport (1, 7).TE materials research primarily focuses on solid and crystalline thermoelectrics. It has been long viewed that all solids contain strong interatomic interactions without even an exception, and thus the established approaches to describe thermal transports in crystalline solids, including TE solids, are solely based on the perturbative "small-parameter" approximation to lattice dynamics of atoms around their equilibrium positions, i.e., phonons and phonon-phonon interactions (10, 11). As a result, crystallographic homogeneity at the atomic level in solid materials has overwhelmingly been accepted. However, recent work on exploring novel TE materials went noticeably beyond the conventional knowledge of solid TE compounds being ideally crystalline, atomically...

Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag8SnSe6

“…These optical phonons barely contribute to the phonon transport due to their very low sound velocities. On the other hand, the scattering channel of the phonon–phonon interaction is greatly enhanced,53, 54 leading to an additional κ L reduction mechanism as reported previously 17. It is thus conclusive that the low sound velocity, together with the low frequencies of the optical phonons, are responsible for the low κ L of Ag 8 SnSe 6 .…”

“…One successful strategy for improving zT is to enhance the power factor S 2 / ρ through band engineering,2, 3, 4, 5, 6, 7 provided the carrier concentration is optimized 8. The other effective strategy is typified by minimizing the only one independent material property, the lattice thermal conductivity ( κ L ), through nanostructuring,9, 10, 11, 12, 13, 14, 15 liquid phonons,16, 17 and lattice anharmonicity 18, 19…”

Intrinsically low thermal conductivity from a quasi-one-dimensional crystal structure and enhanced electrical conductivity network via Pb doping in SbCrSe3

“…54 Such a low optic-mode frequency was also found in the intrinsically low thermal conductivity of the Cu 3 SbSe 3 and CdSb compounds. 9,17 The TA branch exhibited normal behavior with no interactions with the optic-phonon branches, and its frequencies smoothly expanded toward the zone boundary and saturated at~20-40 cm − 1 . The acoustic and low-frequency optic-phonon branches, which were responsible for enhanced scattering and low lattice thermal conductivity, originated from the (Sb-Se) group, as indicated in Figure 8b.…”

“…16 The bond anharmonicity serves as a measure of the deviation in lattice vibrations from a perfect harmonic motion and is characterized by the effective Grüneisen parameter. Generally, structures with a chemical bond hierarchy [17][18][19][20][21] and anisotropic multicenter bonding features 9 or those that possess a lone pair of electrons [22][23][24][25][26] exhibit a stronger anharmonicity. Therefore, the investigation of the chemical bonding, crystal structures and corresponding lattice dynamics is helpful in understanding phonon transport and the design of materials with an intrinsically low lattice thermal conductivity and high thermoelectric performance.…”

Thermoelectric performance enhancement in n-type Bi2(TeSe)3 alloys owing to nanoscale inhomogeneity combined with a spark plasma-textured microstructure

“…[1][2][3][4] The thermoelectric conversion efficiency of a material depends on its dimensionless figure of merit ZT, defined as ZT = α 2 σT/κ, where α, σ, κ and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. 5,6 A great variety of thermoelectric materials have been developed and thoroughly studied, [7][8][9] but industrial applications are still dominated by bismuth telluride (Bi 2 Te 3 )-based alloys. 10 Therefore, extensive studies have been devoted to the enhancement of their properties.…”