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

Abstract

A variety of crystals contain quasi-one-dimensional substructures, which yield distinctive electronic, spintronic, optical and thermoelectric properties. There is a lack of understanding of the lattice dynamics that influences the properties of such complex crystals. Here we employ inelastic neutron scatting measurements and density functional theory calculations to show that numerous low-energy optical vibrational modes exist in higher manganese silicides, an example of such crystals. These optical modes, including unusually low-frequency twisting motions of the Si ladders inside the Mn chimneys, provide a large phase space for scattering acoustic phonons. A hybrid phonon and diffuson model is proposed to explain the low and anisotropic thermal conductivity of higher manganese silicides and to evaluate nanostructuring as an approach to further suppress the thermal conductivity and enhance the thermoelectric energy conversion efficiency. This discovery offers new insights into the structure-property relationships of a broad class of materials with quasi-one-dimensional substructures for various applications.

Optic phonons and anisotropic thermal conductivity in hexagonal Ge2Sb2Te5

“…The magnitude of the anisotropy is similar to that found for layered Bi 2 Te 3 242526 and higher than that of manganese silicide (HMS)27. While significant bonding anisotropy was held responsible for the anisotropic κ in Bi 2 Te 3 28 with van der Waals gaps in the low κ direction, a low-energy (<10 meV) twisting phonon and anisotropic group velocity of the optic phonons were found responsible for the observed κ anisotropy in HMS27. We note that Lee et al 5.…”

MnS Incorporation into Higher Manganese Silicide Yields a Green Thermoelectric Composite with High Performance/Price Ratio

“…At present, significant efforts have been focused on decreasing the κ tot of HMS. Shi and co‐workers19, 20 demonstrated that κ tot can be decreased to 2.2 W m −1 K −1 at 773 K when certain 50–200 nm ReSi 1.75 precipitates formed in the HMS samples; consequently, ZT increased from 0.45 to 0.57. Takeuchi and co‐workers21 proved that κ tot can be further decreased to 1.8 W m −1 K −1 at 773 K when 6.0 at% Re was used to form a supersaturated Re solid solution HMS, and the corresponding ZT value increased to 0.9 at 773 K. In our previous work,22 Te nanowires were selected to prepare MnTe/HMS nano/bulk structures, resulting in 38% reduction of κ tot and ≈71% increase in ZT .…”

Enhancing the Figure of Merit of Heavy‐Band Thermoelectric Materials Through Hierarchical Phonon Scattering

“…Current efforts to improve zT are focused on the optimization of power factor α 2 σ through controlled doping or electron band engineering,7, 8, 9, 10, 11 suppression in κ L by alloying or nanostructuring,1, 2, 5, 6 and development of new materials with intrinsically low κ L 12, 13, 14, 15. Heat‐carrying phonons cover a broad spectrum of frequency, and the κ L is a sum of contributions from phonons of different frequencies.…”