Thermoelectric Properties of Re₃Ge_0.6As_6.4 and Re₃GeAs₆ in Comparison to Mo₃Sb_5.4Te_1.6

Heavily doped narrow gap semiconductors with complex crystal structures are prime candidates for thermoelectric materials. The arsenides Re₃(Ge,As)₇ are new examples with promising thermoelectric properties, comparable with the isostructural Mo₃Sb_5.4Te_1.6, a competitive high-temperature material. Various doping levels may be achieved by using different Ge/As ratios. Re₃(Ge,As)₇ was prepared by heating the elements in the desired ratios in evacuated silica tubes between 600 and 800 °C. Re₃GeAs₆ crystallizes in the cubic Ir₃Ge₇ type, space group Im3̄m, with a = 8.73202(8) Å. It exhibits high Seebeck coefficient, high electrical conductivity, and reasonably low thermal conductivity. Moreover, the thermoelectric figure-of-merit ZT = TS²σ/κ increases rapidly with increasing temperature, as desired for high-temperature thermoelectrics.