The effect of structural vacancies on the thermoelectric properties of (Cu2Te)1−x(Ga2Te3)x

We have studied the effects of structural vacancies on the thermoelectric properties of the ternary compounds (Cu2Te)1−x(Ga2Te3)x (x=0.5, 0.55, 0.571, 0.6, 0.625, 0.667 and 0.75), which are solid solutions found in the pseudo-binary phase diagram for Cu2Te and Ga2Te3, and possesses tunable structural vacancy concentrations. This materials system is not suitable due to the cost and scarcity of the constituent elements, but the vacancy behavior is well understood and will provide a valuable test case for other systems more suitable from the standpoint of cost and abundance of raw materials, which also possesses these vacancy features, but whose structural characterization is lacking at this stage. We find that the nominally defect free phase CuGaTe2 possess the highest ZT (ZT=S2T/ρκ, where S is the Seebeck coefficient and ρ is the electrical resistivity κ is the thermal conductivity and T is the absolute temperature) which approaches 1 at 840 K and seems to continuously increase above this temperature. This result is due to the unexpectedly low thermal conductivity found for this material at high temperature. The low thermal conductivity was caused by strong Umklapp (thermally resistive scattering processes involving three phonons) phonon scattering. We find that due to the coincidentally strong scattering of carriers by the structural defects that higher concentrations of these features lead to poor electrical transport properties and decreased ZT. - Graphical abstract: Thermal conductivity and zT as a function of temperature for a series of compounds of the type (Cu2Te)1-x(Ga2Te3)x (x=0.5, 0.55, 0.571, 0.6, 0.625, 0.667 and 0.75). Highlights: ► All the samples show p-type semiconducting behavior in the temperature dependence of the Seebeck and Hall coefficients. ► The increased carrier concentration and the introduction of vacancies diminish the carrier mobility and power factor. ► The low temperature k decreases significantly as the Ga2Te3 content increases due to increasing point defects. ► The highest ZT ∼ 1.0 at 840 K among the samples in this study was found in CuGaTe2, which contains no vacancies