Search for Resonant-Like Impurity in Ag-Doped CoSb3 Skutterudite: Theoretical and Experimental Study

Abstract

Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) calculations of Ag-doped CoSb3 point to the presence of either an extra sharp peak of s-symmetry Ag density of states near the valence-band edge when filling the void (2a) or to conventional p-type doping when substituting Sb site (24g). These results suggest a resonant-like impurity level in the former or nearly rigid-band behavior in the latter. To confirm the theoretical predictions, a series of samples with nominal composition Co8Sb24:Agx (x = 0, 0.1, 0.3, 0.4, 0.5) were prepared. Structural and phase composition analyses were carried out by x-ray diffraction, scanning electron microscopy, and scanning thermoelectric microprobe. Investigations of the influence of Ag impurity on the electrical conductivity and Seebeck coefficient were performed over the temperature range from 300 K to 560 K. It was found that doping CoSb3 with Ag leads to an increase of the thermoelectric power factor α2σ in the temperature range from 300 K to 475 K of about an order of magnitude for all doped samples. However, electron probe microanalysis revealed accumulation of Ag mainly in grain boundaries while the presence of Ag in CoSb3 crystallites was not confirmed. This observation corroborates the results of KKR-CPA calculations concerning the formation energy of the AgxCo4Sb12 system, which is much lower than values calculated for AxCo4Sb12 (A = Ca, Ba).