Thermoelectric Properties of Doped Rhenium Chalcogenides Re₆ M_x Te₁₅ (x = 0, 1, 2; M = Ga, In, Ag)

Abstract

Temperature dependencies of the electrical resistivity, ρ, and the thermoelectric power, α, are reported for Re₆ M_x Te₁₅ (M = Ga, In, Ag; x = 0, 1, 2) between 90–380 K. Theoretical discussion of the results is presented. The materials, synthesized by filling large voids in the Re₆ Te₁₅ cluster system, may have potential thermoelectric applications around and below room temperatures. The samples are prepared by reacting 99.99% pure elemental powders in evacuated and sealed quartz ampoules at 1070 K for 170 hours. The resistivity data indicate semiconducting behavior for all samples. Possible hopping conduction is present at lower temperatures. The energy gap is observed at higher temperatures in all the samples.

Positive values of α in Re₆ (Ga,In)_x Te₁₅ (x = 0, 1, 2) indicate p-type semiconducting behavior in the studied temperature range. For these samples α increases initially with temperature, then levels off to a nearly constant value. The positions of the sharp peaks in a, observed at lower temperatures for x = 1, 2 only, depend on the Ga (In) concentration. High values of a (∼ 300 μV/K) are measured at room temperatures. In Re₆ AgTe₁₅ α has small positive values (∼ 20–40 μV/K) between 185 K and 270 K. Outside this range α is negative. It reaches local maxima of -340 μV/K at 105 K and -350 μV/K at 370 K. In Re₆ Ag₂ Te₁₅ α changes from positive to negative values above 295 K. A maximum positive value of +350 μV/K is reached at 250 K and maximum negative of -250 μV/K at 330 K. The power factor, α²/ρ, increases with temperature for all studied samples. Theoretical fits to α(T) for all samples are discussed. Also discussed is the effect of filling the voids in the rhenium-telluride system on the figure of merit.