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Electrical and thermoelectric properties of nanocrystal substitutional semiconductor alloys Mg3(BixSb1−x)2 prepared by mechanical alloying

Nanocrystal substitutional semiconductor alloys Mg3(BixSb1−x)2 (nano-Mg3(BixSb1−x)2) with a mean grain size of ∼30 nm were prepared by mechanical alloying plus hot-pressing, and their dc electrical and thermoelectric properties were investigated from room temperature down to 20 K. The results indicated that lattice parameters a and c of nano-Mg3(BixSb1−x)2 increased linearly with increasing Bi content x, in agreement with Vegard's law. The dc resistivity ρ of nano-Mg3(BixSb1−x)2 decreased monotonically with increasing x, and a drop of over five orders of magnitude was reached at 300 K when x increased from 0 to 1. Moreover, the temperature behaviour of the resistivity of nano-Mg3(BixSb1−x)2 changed sensitively with x, and a transition from the semiconducting state (i.e. dρ/dT < 0) to the metallic state (dρ/dT > 0) occurred between x = 0.7 and 0.8. Meanwhile, this transition was verified by the measurements of the temperature behaviour of the Seebeck coefficient S of nano-Mg3(Bi1−xSbx)2 with different x. In addition, Mott's ρ ∝ T−1/4 law was observed at lower temperature regimes for the nano-Mg3(BixSb1−x)2 (x ≠ 0), suggesting the occurrence of hopping conduction. Although experiments showed that the Seebeck coefficient of nano-Mg3(Bi1−xSbx)2 decreased monotonically with x, their thermoelectric power factors PF changed non-monotonically, and a maximum PF of 1.4 µW cm−1 K−2 was achieved at room temperature for x = ∼0.8, which was more than three orders magnitude greater than that of monolithic Mg3Sb2.