Materials and Methods 1. Sample preparation method Tl-doped PbTe was made by direct reaction of appropriate amounts of Pb, Te, and Tl 2 Te in a fused-silica tube sealed under a vacuum. Each sample was briefly melted at 1273 K for 24 h and lightly shaken to ensure homogeneity of the liquid, then furnace cooled to 800 K and annealed for 1 week. The obtained ingot was crushed into fine powder and hot-pressed at 803 K for 2 hours under a flowing 4% H 2-Ar atmosphere. The final form of each polycrystalline sample was a 2mm thick disk about 10 mm in diameter. Phase purity was checked by powder X-ray diffraction. No impurity phases were found in the XRD patterns, indicating that all added Tl was dissolved in PbTe. The purities of all starting materials were at least 99.99%. The samples were stable in air at room temperature.

We report measurements of the thermal conductivity of high-quality crystals of the cubic I-V-VI2 semiconductors AgSbTe2 and AgBiSe2. The thermal conductivity is temperature independent from 80 to 300 K at a value of approximately 0.70 W/mK. Heat conduction is dominated by the lattice term, which we show is limited by umklapp and normal phonon-phonon scattering processes to a value that corresponds to the minimum possible, where the phonon mean free path equals the interatomic distance. Minimum thermal conductivity in cubic I-V-VI2 semiconductors is due to an extreme anharmonicity of the lattice vibrational spectrum that gives rise to a high Grüneisen parameter and strong phonon-phonon interactions. Members of this family of compounds are therefore most promising for thermoelectric applications, particularly as p-type materials.

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