Filling fraction of Yb in CoSb₃ Skutterudite studied by electron microscopy

The filling fraction limit in filled Skutterudites plays a critical role in thermoelectric properties; however, the exact filling fraction limit for certain fillers is still in debate. In this work, we observed the lattice distortion, where lattice parameters a and b are unequal, in the annealed Yb-filled CoSb₃. Microstructure characterization by advanced spherical aberration-corrected (C_S-corrected) electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy in conjunction with thermoelectric characterization clearly proves that the Yb filling fraction is higher in the annealed sample (with lattice distortion) than in the ball-milled sample (without lattice distortion). Therefore, the results indicate that lattice distortion appears when the Yb filling fraction reaches a certain critical value in CoSb₃ Skutterudites. The observed lattice distortion provides an alternative approach to probe the filling fraction in filled Skutterudites.