Enhancement of thermoelectric performance by phase seperation of Ag$_{2}$Te in quaternary Ag$_{\mathrm{x}}$Bi$_{0.5}$Sb$_{\mathrm{1.5-x}}$Te$_{\mathrm{3-x}}$

Quaternary Ag--Bi--Sb--Te alloys with the general formula of Ag$_{x}$Bi$_{0.5}$Sb$_{1.5-x}$Te$_{3-x}$ are synthesized by solid state reaction for the high Ag doping x$=$0.1, 0.2, and 0.3. The powder x-ray diffraction analysis of the melted ingot shows the phase separation of AgSbTe$_{2}$ and Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ phases. After the hot press sintering at 350 ${^\circ}$, we found Ag$_{2}$Te/Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ composite, instead of AgSbTe$_{2}$ phase separation, from the energy dispersive x-ray spectroscopy and x-ray diffraction measurements. The electrical conductivities of the Ag$_{2}$Te/Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ composite are significantly increased comparing with that of conventional $p$-type Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ compound, implying that the interface effect by phase separation can attribute to the increase of electrical conductivity. The maximum power factor and \textit{ZT} values are reached up to 2.1 mW K$^{-2}$ m$^{-1}$ ($\sim$ 400 K) and 1.1 (at 570 K), respectively, for x $=$ 0.1 composite.. Here we propose that the phase separation of Ag$_{2}$Te in Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ matrix can increase thermoelectric performance at mid-temperature temperature range.