Infrared study on the electronic structure of the alkaline-earth-filled skutterudites AM₄Sb₁₂ (A=Sr, Ba; M=Fe, Ru, Os)

The optical conductivity [σ(ω)] spectra of alkaline-earth-filled skutterudites with the chemical formula A²⁺M₄Sb₁₂ (A=Sr, Ba; M=Fe, Ru, Os) and a reference material La³⁺Fe₄Sb₁₂ were obtained and compared with the corresponding band-structure calculations and with calculated σ(ω) spectra to investigate their electronic structures. At high temperatures, the energy of the plasma edge decreased with the increasing valence of the guest atoms A in the Fe₄Sb₁₂ cage indicating hole-type conduction. A narrow peak at 25 meV with a pseudogap was observed in SrFe₄Sb₁₂, while the corresponding peaks were located at 200 and 100 meV in the Ru and Os counterparts, respectively. The order of the peak energy in these compounds is consistent with the thermodynamical properties in which the Os compound is located between the Fe and Ru compounds. This indicated that the electronic structure observed in the infrared σ(ω) spectra directly affects the thermodynamical properties. The band-structure calculation implies that the different electronic structures among these compounds originate from the different d states of the M ions.