Infrared study on the electronic structure of alkaline-earth-filled skutterudites AT4Sb12 (A = Sr, Ba; M = Fe, Ru, Os)

TL;DR

This study investigates the electronic structures of alkaline-earth-filled skutterudites using infrared spectroscopy and band structure calculations, revealing how guest atom valence and $d$ states influence their optical and thermodynamical properties.

Contribution

It provides new insights into the relationship between electronic structure and thermodynamical behavior in skutterudites through combined experimental and theoretical analysis.

Findings

Plasma edge energy decreases with increasing guest atom valence.

A pseudogap of 25 meV observed in SrFe4Sb12.

Electronic structure differences originate from $d$ states of M ions.

Abstract

The optical conductivity [$\sigma(\omega)$] spectra of alkaline-earth-filled skutterudites with the chemical formula $A^{2+}M_{4}$Sb$_{12}$ ($A$ = Sr, Ba, $M$ = Fe, Ru, Os) and a reference material La$^{3+}$Fe$_{4}$Sb$_{12}$ were obtained and compared with the corresponding band structure calculations and with calculated $\sigma(\omega)$ 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 Fe$_{4}$Sb$_{12}$ cage indicating hole-type conduction. A narrow peak with a pseudogap of 25 meV was observed in SrFe$_{4}$Sb$_{12}$, while the corresponding peak 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 $\sigma(\omega)$ spectra directly affects the thermodynamical properties. The band structure calculation implies the different electronic structure among these compounds originates from the different $d$ states of the $M$ ions.