The absolute S value of Ag2Se drops from about 110 μV/K at room temperature to 34 μV/K around 403K, and then slightly increases to 37 μV/K with increasing temperature to 573K. The temperature-dependent trend in S are totally in the same direction of ρ. Because the S value decreases with carrier concentration, this abrupt decrease in S values around 403K is due to the largely increased carrier concentration (Fig. 3 (b)) as the result of the semiconductor−superionic conductor phase transformation.