Role of anharmonic strength and number of allowed three-phonon processes in lattice thermal conductivity of SnTe based compounds

Abstract

The lattice heat transport properties of the thermoelectric (TE) material SnTe and the doped Sn7SbTe8 and Sn7BiTe8 are examined using Boltzmann transport theory supplemented with first-principle calculations. We illustrate the microscopic origin of the lattice thermal conductivity, κ l of the materials by calculating the mode Grüneisen parameters, phase space volume for three-phonon processes, the anharmonic scattering rates (SR), and the phonon group velocities. SnTe is found to be a low κ l material with a value of ∼3 W mK−1 at room temperature in agreement with experiments. The phonon scatterings in pristine SnTe mainly originates in the strong anharmonicity of the material, as evidenced by the large values of its mode Grüneisen parameters. Doping with Sb or Bi reduces the anharmonic strength. For Sb doped Sn7SbTe8, it results in a drop in the SR and hence a higher κ l value. However in the Bi doped Sn7BiTe8, the number of allowed three-phonon processes gets greatly enhanced which compensates for the reduction in anharmonicity. This coupled with lower phonon group velocities lowers the κ l value for the Bi doped system below that of pristine SnTe. In nanowire structures, κ l values for the doped systems get drastically reduced yielding an ultra-low value of 0.84 W mK−1 at 705 K for the Bi doped material for a nanowire of 10 nm diameter.

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“…This is contrary to our previous study on TiCoBi and TiRhBi, 35 where W was significantly larger in TiRhBi than in TiCoBi, indicating that the Rh substitution at the Co site strongly enhances the number of three-phonon processes for each phonon. It is shown that while the anharmonic scattering rate in pristine SnTe is caused mainly by the anharmonic strength, in Bi/Sb-doped SnTe, 43 where the anharmonicity is reduced, the number of allowed threephonon processes plays a substantial role in the scattering rates.…”