Exploring the Role of Interstitial Occupation of Cobalt on the Thermoelectric Properties of Co-Excess Te-Substituted Skutterudite CoSb ₃

The introduction of excess cobalt in skutterudite CoSb3 causes the excess cobalt atoms to preferentially occupy an interstitial position over the conventional lattice sites, enabling additional phonon scattering mechanisms in addition to mass fluctuations and rattlers, which collectively help in fine-tuning the thermal conductivity. The presence of the interstitial cobalt atom is reflected in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image viewed along the [111] axis of the Co1.02Sb3 sample. This occupation of interstitial cobalt is also detected by the introduction of additional Raman active modes (123.82 and 164.38 cm–1) observed by Raman spectroscopy. The electronic, structural, and vibrational properties of pristine, Co-excess, and Te-substituted Co-excess CoSb3 are studied using density functional theory. The Co-excess sample Co1.02Sb3 displays a p-type semiconducting nature with an enhanced Seebeck coefficient compared to that of pristine CoSb3, driven by the conventional and unconventional band convergence present in the valence band. Te substitution into Co1.02Sb3 forms an n-type degenerate semiconductor, accompanied by the emergence of strong Co 3d and Te 5p orbital interactions as revealed by crystal orbital Hamilton population analysis, exhibiting a significant antibonding state just below the Fermi level. The introduction of additional vibrational modes due to interstitial cobalt, concurrent with the mass fluctuations in Te-substituted Co1.02Sb3, maintains a low thermal conductivity (3.60 W/m·K at 675 K). The maximum zT achieved is 0.76 at 773 K for the composition Co1.02Sb2.90Te0.10. X-ray photoelectron spectroscopy studies on a cleaned and desiccated sample revealed the oxidation of the sample surface at room temperature to form native oxides Sb2O3 and Co(OH)2. Etching the sample surface by 41 nm effectively removed the oxides, enabling accurate analysis of the chemical state of elements.