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Lattice Thermal Conductivity of NiTiSn Half-Heusler Thermoelectric Materials from First-Principles Calculations

The microscopic physics behind the lattice thermal conductivity of NiTiSn is investigated using first-principles-based anharmonic lattice dynamics. The calcu lated lattice thermal conductivity of bulk materials (5.3 W/m.K) is in good agreement with the experimental value at the optimal working temper ature (700 K), but is overestimated below this temperature. The calculated values can be strongly affected by the size of the crystalline grains. We show tha t the lattice thermal conductivity is dominated by the acoustic (transverse and mostly longitudinal) modes with no contribution from the optical modes. The a coustic phonons are located below 150 cm-1 and involve mainly the tin atoms. The calculated mean free path of the most heat carrying phonons is around f ifty nanometers with a maximum life time of approx. 100 ps. These theoretical results are a step forward in developing the experimental design of low thermal conductivity NiTiSn Heusler based materials.