Significantly reduced lattice thermal conductivity with anharmonic lattice vibrations and band edge effect in electronic thermal conductivity in Ag₂S_1-xSe_x (0 ≤ x ≤ 0.6)

The composition dependence of the unusual behaviors in electronic and lattice thermal conductivity, κele and κlat, in Ag2S1- xSe x ( x = 0–0.6 in steps of 0.1, 300 K) is investigated in detail by means of precise electron and heat transport properties measurement, synchrotron X-ray crystal structure and electron density distribution analyses, and sound velocity measurement. We reveal that the κele of Ag2S1- xSe x is strongly affected by the fine electronic structure of the conduction band edge near the chemical potential and the thermoelectric motive force; therefore, these effects make the κele of Ag2S1- xSe far different from that calculated by the Wiedemann–Franz law, κele = L0σ T, with the Lorentz number L0 = π2 kB2/(3 e2). It is also clearly demonstrated that the κlat of Ag2S1- xSe x is greatly reduced by anharmonic lattice vibrations and that the magnitude of κlat is quantitatively reproduced by an equation representing the thermal conductivity under the strongest scattering limit. The κlat decreases with increasing x and saturates at 0.4 W m−1 K−1 at x ≥ 0.4. This is caused by the increasing anharmonic lattice vibrations with x, and its saturating behavior is determined by the strongest scattering limit. On the other hand, a negligibly small κele at x = 0 turns out to be non-trivial at x ≥ 0.4 owing to the increasing carrier density with x, most likely contributed by the increasing interstitial Ag defects. Consequently, the total thermal conductivity of Ag2S1− xSe x becomes minimum not at x = 0.5 (composition of the maximum structure entropy) but at x = 0.3.