While adding extra Mg is intended to compensate for the Mg loss during the synthesis, it often leads to Mg interstitials in Mg 2 (Si,Sn) materials and profoundly affects their thermoelectric properties. Herein we studied the electrical conductivity, Seebeck coefficient, and thermal conductivity of Mg 2(1+x) Si 0.38 Sn 0.6 Sb 0.02 (0.05 # x # 0.12) as a function of Mg excess between 300 K and 730 K. The presence of interstitial Mg was corroborated by X-ray powder diffraction, X-ray photoelectron spectroscopy, Hall coefficient measurement, and compositional analysis. The electrical properties have been analyzed in the framework of a single parabolic band model to gain more insight on the roles of Mg interstitials. We found that increasing Mg excess content increased the carrier concentration, electronic effective mass, and electrical conductivity, while it decreased the Seebeck coefficient and led to a non-monotonic change in the lattice thermal conductivity. As a result, a maximum ZT $ 0.85 was attained at 700 K for Mg excess x ¼ 0.1, a $60% enhancement compared to that of the sample x ¼ 0.05. The Mg interstitials thus provide an extra tuning parameter in optimizing the thermoelectric properties of Mg 2 (Si,Sn)-based materials.

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Through coordination of the Seebeck coefficient and carrier concentration in Cu3SnS4, TE performance improves significantly with the ZT value of 0.75 at 790 K.

I-III-VI2 chalcopyrites have unique inherent crystal structure defects, and hence are potential candidates for thermoelectric materials. Here, we identified mixed polyanionic/polycationic site defects (ZnIn(-), VCu(-), InCu(2+) and/or ZnCu(+)) upon Zn substitution for either Cu or In or both in CuInTe2, with the ZnIn(-) species originating from the preference of Zn for the cation 4b site. Because of the mutual reactions among these charged defects, Zn substitution in CuInTe2 alters the basic conducting mechanism, and simultaneously changes the lattice structure. The alteration of the lattice structure can be embodied in an increased anion position displacement (u) or a reduced bond length difference (Δd) between d(Cu-Te)4a and d(In-Te)4b with increasing Zn content. Because of this, the lattice distortion is diminished and the lattice thermal conductivity (κL) is enhanced. The material with simultaneous Zn substitution for both Cu and In had a low κL, thereby we attained the highest ZT value of 0.69 at 737 K, which is 1.65 times that of Zn-free CuInTe2.

In this project, we have successfully manipulated the lattice defects in α-In 2 Se 3 -based solid solutions (In 2-x Zn x Se 3 ) upon proper substitutions of Zn for In, via a non-equilibrium fabrication technology of materials (NEFT). The manipulation of the defects centers on reducing the number of interstitial In atoms (In i ) and Se vacancies (V Se ), and creating a new antisite defect Zn In as a donor. By such means, the lattice structure tends to be ordering, and also more stabilized than that of pure α-In 2 Se 3 . At the meantime, the carrier concentration (n) and mobility (µ) have increased by 1~2 orders of magnitude. As a consequence, the solid solution at x=0.01 gives the 10 highest TE figure of merit (ZT) of 1.23(±0.22) in the pressing direction at 916K, which is about 4.7 times that of virgin α-In 2 Se 3 (ZT=0.26). This achieved TE performance is mainly due to the remarkable improvement of the electrical conductivity that increases from 0.53×10 3 (Ω -1 m -1 ) at x=0 to 4.88×10 3 (Ω -1 m -1 ) at x=0.01 at 916K, in spite of the enhancement of the lattice thermal conductivity (κ L ) from 0.26 (wm -1 k -1 ) to 0.32 (wm -1 k -1 ).