CoSb3-based filled skutterudite has emerged as one of the most viable candidates for thermoelectric applications in automotive industry. However, the scale-up commercialization of such materials is still a challenge due to the scarcity and cost of constituent elements. Here we study Ce, the most earth abundant and low-cost rare earth element as a single-filling element and demonstrate that, by solubility design using a phase diagram approach, the filling fraction limit (FFL) x in CexCo4Sb12 can be increased more than twice the amount reported previously (x=0.09). This ultra-high FFL (x=0.20) enables the optimization of carrier concentration such that no additional filling elements are needed to produce a state of the art n-type skutterudite material with a zT value of 1.3 at 850 K before nano-structuring. The earth abundance and low cost of Ce would potentially facilitate a widespread application of skutterudites.

Heavy doping changes an intrinsic semiconductor into a metallic conductor by the introduction of impurity states. However, Ga impurities in thermoelectric skutterudite CoSb3 with lattice voids provides an example to the contrary. Because of dual‐site occupancy of the single Ga impurity charge‐compensated compound defects are formed. By combining first‐principle calculations and experiments, we show that Ga atoms occupy both the void and Sb sites in CoSb3 and couple with each other. The donated electrons from the void‐filling Ga (GaVF) saturate the dangling bonds from the Sb‐substitutional Ga (GaSb). The stabilization of Ga impurity as a compound defect extends the region of skutterudite phase stability toward Ga0.15Co4Sb11.95 whereas the solid–solution region in other directions of the ternary phase diagram is much smaller. A proposed ternary phase diagram for Ga‐Co‐Sb is given. This compensated defect complex leads to a nearly intrinsic semiconductor with heavy Ga doping in CoSb3 and a much reduced lattice thermal conductivity (κL) which can also be attributed to the effective scattering of both the low‐ and high‐frequency lattice phonons by the dual‐site occupant Ga impurities. Such a system maintains a low carrier concentration and therefore high thermopower, and the thermoelectric figure of merit quickly increases to 0.7 at a Ga doping content as low as 0.1 per Co4Sb12 and low carrier concentrations on the order of 1019 cm−3.

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 x Co 4 Sb 12Àx/3 with x varying from 0.075 to 0.6 and In 0.3 Co 4Ày Sb 11.9+y with y changing from À0.20 to 0.20) are synthesized and characterized. X-ray analysis and scanning electron microscopy images indicate that, up to x ¼ 0.27, single-phase skutterudites are obtained with lattice constant increasing with In fraction x. A fixed-composition skutterudite In 0.27AE0.01 Co 4 Sb 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 4 Sb 11.9 at 750 K. Broader contextIn-containing skutterudites have long attracted much attention due to the high thermoelectric efficiency and proposed application in automotive waste heat recovery. However there has been much debate over how much indium is actually soluble in CoSb 3 or whether it simply precipitates out as nanoparticles. In this study, we provide both theoretical and experimental evidence for the presence of compound defects in skutterudites with indium impurities. The isothermal section of ternary phase diagram of In-Co-Sb system at 873 K is proposed, which naturally explains the solubility debate and the confusion concerning the various reports of maximum solubility of indium in the skutterudite phase. The phase relations allow us to identify a stable skutterudite composition In 0.27 Co 4 Sb 11.9 with high zT values greater than 1. A wide range of nominal compositions will produce the same skutterudite material as the majority phase which will enable the exibility needed to produce commercial quantities of reliable, uniform quality.

Although filled skutterudites are actively being developed for automotive waste heat recovery the compositions considered available are limited. Typically synthesis conditions are chosen with slight excess filling element to produce filled skutterudites at the 'filling fraction limit', traditionally considered to be a single value that depends only on the elements involved (e.g. filling element, Co/Fe ratio). The filling fraction limit is often debated and thought perhaps to vary with processing method. This work opens up a new dimension of available compositions by showing that the 'filling fraction limit' is a thermodynamic quantity that for Yb varies by a factor of 5 depending on annealing temperature and nominal composition. As the filling element controls the electronic doping in these semiconductors this study not only enables optimization of thermoelectric properties using thermodynamic control rather than non-equilibrium processing conditions (reaching zT ¼ 1.3 at 850 K without nanostructures) but it also predicts dopant precipitation effects after extended use. The novel phase diagram approach used here should be easily applied to other ternary thermoelectric materials to uncover similar phenomena. Thus skutterudite material with the optimized thermoelectric composition can be produced from a range of nominal compositions with appropriate annealing.

Sn-Sb alloys are important high-temperature solders. However, inconsistencies are found in the available phase diagrams, and some phase boundaries in the Sn-Sb system have not been determined. Sn-Sb alloys were prepared, equilibrated at 160°C to 300°C, and the equilibrium phases and their compositions were determined. The b-SnSb phase has a very wide compositional homogeneity range, and its composition varies from Sn-47.0at.%Sb to Sn-62.8at.%Sb. There is no order-disorder transformation of the b-SnSb phase. There are three peritectic reactions in the Sn-Sb system, L + Sb = b-SnSb, L + b-SnSb = Sn 3 Sb 2 , and L + Sn 3 Sb 2 = Sn, and their temperatures are 424°C, 323°C, and 243°C, respectively. Thermodynamic models of the Sn-Sb binary system were developed using the CALPHAD approach based on the experimental results of this study and the data in the literature. The calculated phase diagram and thermodynamic properties are in good agreement with the experimental determinations.