Phase diagram of In–Co–Sb system and thermoelectric properties of In-containing skutterudites

Abstract

In-containing skutterudites have long attracted much attention and debate partly due to the solubility limit issue of indium in CoSb_3. The isothermal section of the equilibrium phase diagram for the In–Co–Sb system at 873 K is proposed using knowledge of the related binary phase diagrams and experimental data, which explains the debated indium solubility that depends on Sb content. In this paper, a series of In-containing skutterudite samples (In_xCo_4Sb_(12−x/3) with x varying from 0.075 to 0.6 and In_(0.3)Co_(4−y)Sb_(11.9+y) with y changing from −0.20 to 0.20) are synthesized and characterized. X-ray analysis and scanning electron microscopy images indicate that, up to x = 0.27, single-phase skutterudites are obtained with lattice constant increasing with In fraction x. A fixed-composition skutterudite In_(0.27±0.01)Co_4Sb_(11.9) was determined for the Co-rich side of In–CoSb_3 which is in coexistence with liquid InSb and CoSb_2. Indium, like Ga, is expected, from DFT calculations, to form compound defects in In-containing skutterudites. However, relatively higher carrier concentrations of In-containing skutterudites compared to Ga-containing skutterudites indicate the existence of not fully charge-compensated compound defects, which can also be explained by DFT calculations. The net n-type carrier concentration that naturally forms from the complex defects is close to the optimum for thermoelectric performance, enabling a maximum zT of 1.2 for the fixed skutterudite composition In_(0.27)Co_4Sb_(11.9) at 750 K.

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