Assessment of Hot-Pressing Sintering Effect of Skutterudite In0.40Mn0.15Co3.85Sb12, Structure, Optical, and Electrical Properties

In this study, In0.40Mn0.15Co3.85Sb12 was synthesized by the ceramic method, using a traditional melting–annealing treatment (MA), followed by grinding and sintering via the hot-pressing (HP) technique. Rietveld refinement of the powder diffraction (PXRD) data confirms that the resulting phase has a cubic crystal structure in space group Im-3, which is isostructural with the pristine Co4Sb12 phase. The cell parameter a of the filled In0.40Mn0.15Co3.85Sb12 increases after hot pressing compared with the Co4Sb12 phase. This suggests that the partial substitution of cobalt atoms with manganese (Mn) alters the cell size of the resulting material. The PXRD pattern of the In0.40Mn0.15Co3.85Sb12 phase of the MA sample shows a low-intensity line (~30°), which is related to elemental antimony (~4%, by Rietveld refinement). Rietveld refinements support a second model which implies the pressure-induced self-insertion of remanent antimony from the (MA) phase into the void sites after (HP) treatment, leading to a new phase: In0.30Sb0.10Mn0.15Co3.85Sb11.90 (HP). The vibrational Raman modes of the obtained phases, In0.40Mn0.15Co3.85Sb12 (MA and HP), are correlated with those of the pristine phase, Co4Sb12. A strong primary signal at 185 cm−1 in the Raman spectrum of In0.40Mn0.15Co3.85Sb12 (MA) is associated with antimony impurities, which is confirmed by Rietveld refinement. Raman spectra of the HP sample are well correlated to the (SPS) Co4Sb12 phase, which reveals structural changes due to self-insertion of antimony into the voids. The band-gap energy values of both the In0.40Mn0.15Co3.85Sb12 (MA) phase and the (HP) phase are 0.750 ± 0.006 eV and 0.650 ± 0.004 eV, respectively. These values are higher than those of the Co4Sb12 phase, which has a band-gap energy of 0.55 eV. This indicates that the electronic band structure is modified by the partial substitution of cobalt with manganese and the introduction of indium in the icosahedral cages. Electrical transport properties at room temperature show that In0.40Mn0.15Co3.85Sb12 (MA) and In0.30Sb0.10Mn0.15Co3.85Sb11.90 (HP) are n-type semiconductors.