Recently, researchers from Institute of Solid State Physics, Hefei Institutes of Physical Science, constructed two concise parameters in characterization of thermal conductivity of half-Heusler compounds.
Thermoelectric material is one kind of functional materials which can directly convert heat into electricity. Understanding the factors that affect the thermal conductivity is a topic of great scientific interest, which will guide to reduce the lattice thermal conductivity or search materials with low lattice thermal conductivity.
Key developments have been the concept of the phonon-glass-electron crystal (PGEC) and the related idea of rattling to achieve this.
The structure for half-Heusler (HH) compounds consists of three interpenetrating face centered cubic (fcc) sublattices and one vacant fcc sublattice, which also can be viewed as a filled zinc blende lattice. Therefore, if one of the atoms is weakly bound, one could imagine that it may serve as a rattler lowering the thermal conductivity.
It is the crucial issue to explore a better understanding of rattling in relation to thermal conductivity using this unique HH structure, to look for and design new HH material with the low lattice thermal conductivity.
By using first-principles phonon and thermal conductivity calculations, the team explored the concept of rattling for HH compounds.
They found that the thermal conductivity was correlated with average phonon frequency as expected and also surprisingly well with the average effective spring constant.
They constructed two measures based on local dynamics using the site average phonon frequency from the projected phonon density of states. The first was a ratio of the lowest site average frequency to the highest (ωmin/ωmax). The second was a ratio of the lowest effective spring constant to the highest (kmin/kmax).
The two parameters were correlated to the thermal conductivity. The material had small ωmin/ωmax and kmin/kmax suggested it had the weak bonding between atoms and the low rattling frequency. This could determine materials with low lattice thermal conductivity simply and effectively.
The result may not only provide understanding of rattling in relation to thermal conductivity but also offer new insights into discovering and designing materials with low thermal conductivity.
The crystal structure of half-Heusler material. (Image by FENG Zhenzhen)
(a) The ratio of kmin to kmax, here kmin and kmax are the smallest and largest spring constants among three atoms, respectively. (b) the ratio of ωmin to ωmax, here ωmin and ωmax are the smallest and largest angular frequency among three atoms. (c) Calculated mean square displacement (MSD) at 300 K vs effective spring constant for the atoms in our dataset. Labels denote specific atoms in the compounds in parentheses. (d) Calculated the anharmonic scattering rates at 300 K for the four low thermal conductivity compounds. (Image by FENG Zhenzhen)
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