Skutterudites CoSb(3) with multiple cofillers Ba, La, and Yb were synthesized and very high thermoelectric figure of merit ZT = 1.7 at 850 K was realized. X-ray diffraction of the densified multiple-filled bulk samples reveals all samples are phase pure. High-resolution scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) analysis confirm that multiple guest fillers occupy the nanoscale-cages in the skutterudites. The fillers are further shown to be uniformly distributed and the Co-Sb skutterudite framework is virtually unperturbed from atomic scale to a few micrometers. Our results firmly show that high power factors can be realized by adjusting the total filling fraction of fillers with different charge states to reach the optimum carrier density, at the same time, lattice thermal conductivity can also be significantly reduced, to values near the glass limit of these materials, through combining filler species of different rattling frequencies to achieve broad-frequency phonon scattering. Therefore, partially filled skutterudites with multiple fillers of different chemical nature render unique structural characteristics for optimizing electrical and thermal transports in a relatively independent way, leading to continually enhanced ZT values from single- to double-, and finally to multiple-filled skutterudites. The idea of combining multiple fillers with different charge states and rattling frequencies for performance optimization is also expected to be valid for other caged TE compounds.

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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.