Phase stability, crystal structure and thermoelectric properties of Cu doped AgSbTe2

CuxAg1-xSbTe2 samples withx = 00.3 are prepared by a combined process of melt-quenching and spark plasma sintering (SPS). X-ray powder diffraction (XRD) analysis indicates that single phase samples with the NaCl-type structure are obtained for the Cu-doped samples before SPS treatment, whereas a small quantity of Ag2Te impurities coexist with the main cubic phase for the sample without Cu. According to our thermoanalysis and XRD results, the substitution of Cu for Ag can effectively prevent the precipitation of Ag2Te, but this also leads to the presence of a minor amorphous phase in the melt-quenched sample. The amorphous phase crystallizes into Sb7Te metastable phase at about 540 K, which finally transforms into the stable Sb2Te3 compound. After the SPS treatment of the melt-quenched sample, the sample withx=0.1 remains a single phase with the face-centered-cubic crystal structure, while Sb7Te and Sb2Te3 are precipitated as the second phases for the samples withx = 0.2 and 0.3, respectively. The electrical conductivity increases and the Seebeck coefficient decreases with the addition of Cu due to the existence of the second phase in the samples withx = 0.2 and 0.3. Accordingly, thermal conductivities also increase with the addition of Cu, leading to the reduced thermoelectric performance of thex= 0.2 and 0.3 samples. For the sample withx = 0.1, its power factor is comparable to that of the literature reported AgSbTe2 compound. As a result of so-called alloying effect, the phonon scattering effect is enhanced due to the partial replacement of Ag by Cu, leading to the reduced thermal conductivity of thex = 0.1 sample. Therefore, the Cu0.1Ag0.9SbTe2 sample exhibits the promising thermoelectric performance and a dimensionless thermoelectric figure of merit (ZT) value of 1 is achieved at 620 K.