Thermoelectric has long been recogn

Thermoelectric has long been recognized as a potentially transformative energy conversion technology due to its ability to convert heat directly into electricity. However, how to optimize the three interdependent thermoelectric parameters (i.e., electrical conductivity sigma, Seebeck coefficient S, and thermal conductivity kappa) for improving thermoelectric properties is still challenging. Here, we put forward for the first time the semiconductor-superionic conductor phase transition as a new and effective way to selectively optimize the thermoelectric power factor based on the modulation of the electric transport property across the phase transition. Ultra low value of thermal conductivity was successfully retained over the whole investigated temperature range through the reduction of grain size. As a result, taking monodisperse Ag2Se nanocrystals for an example, the maximized ZT value can be achieved around the temperature of phase transition. Furthermore, along with the effective scattering of short-wavelength phonons by atomic defects created by alloying, the alloyed ternary silver chalcogenide compounds, monodisperse Ag4SeS nanocrystals, show better ZT value around phase transition temperature, which is cooperatively contributed by superionic phase transition and alloying at nanoscale.