Atomic Structure and Lattice Dynamics of CoSb3 Skutterudite-Based Thermoelectrics

The local atomic structure of skutterudite-type compounds derived from CoSb3 plays a pivotal role in tuning their electronic and thermal properties in thermoelectric applications. For instance, the shape of the occurring [Sb4] rings has direct consequences on the band convergence and, then, the possible enhancement of the thermoelectric efficiency. In this work, both local and electronic structures of the CoSb3 skutterudite were evaluated by the X-ray absorption technique. Extended X-ray-absorption fine-structure oscillations at the Sb K-edge were fitted in good agreement to the body-centered cubic phase. The edge shift values were taken referencing the Co and Sb foils. The standard samples were used, namely, CoO (Co2+), Co3O4 (Co2.5+), Sb2O3 (Sb3+), and Sb2O5 (Sb5+). Based on the valence state dependence of the edge shift, the valences of Co and Sb in CoSb3 were estimated as +0.8(5) for Co and −2.2(3) for Sb, which suggests a partial charge transfer from the metal to the pnictide element. From the bonding distances of Co–Sb, Sb–Sb (short), and Sb–Sb (long), the lattice parameter and fractional coordinates (y, z) were evaluated and, then, compared to those extracted from synchrotron X-ray diffraction. From temperature-dependent X-ray absorption spectroscopy data at 80–350 K, the Einstein temperatures and local coefficients of thermal expansion of those pair-bonds were properly estimated. Comparing these values with those obtained from diffraction, we have established the boundaries of both short- and long-range order techniques for structural characterization of skutterudite-based thermoelectrics.