The effect of structural vacancies on the thermoelectric properties of (Cu₂Te){sub 1–x}(Ga₂Te₃)x

• Published in: Journal of solid state chemistry Vol. 201
• Main Authors: Ye, Zuxin, Young Cho, Jung, Tessema, Misle M., Salvador, James R., Waldo, Richard A., Wang, Hsin, Cai, Wei
• Format: Journal Article
• Language: English
• Published: United States 01.05.2013
• Subjects: CARRIER MOBILITY; CHALCOPYRITE; COPPER TELLURIDES; ELECTRIC CONDUCTIVITY; GALLIUM TELLURIDES; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; PHASE DIAGRAMS; PHONONS; RAW MATERIALS; SCATTERING; SOLID SOLUTIONS; TEMPERATURE DEPENDENCE; THERMAL CONDUCTIVITY; THERMOELECTRIC PROPERTIES
• DOI: 10.1016/J.JSSC.2013.02.008
• ISSN: 0022-4596

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.