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

In-containing skutterudites have long attracted much attention and debate partly due to the solubility limit issue of indium in CoSb₃. 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_xCo₄Sb_12-x/3 with x varying from 0.075 to 0.6 and In_0.3Co_4-ySb_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±0.01Co ₄Sb_11.9 was determined for the Co-rich side of In-CoSb₃ 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_0.27Co₄Sb_11.9 at 750 K.