Vibrational and structural properties of the $R$Fe$_{4}$Sb$_{12}$ ($R=$Na, K, Ca, Sr, Ba) filled skutterudites

TL;DR

This study investigates the vibrational and elastic properties of $R$Fe$_{4}$Sb$_{12}$ skutterudites with different fillers using temperature-dependent EXAFS and pressure-dependent XRD, revealing insights into filler-cage interactions and mechanical properties.

Contribution

It provides new experimental data on the vibrational and elastic properties of various $R$Fe$_{4}$Sb$_{12}$ skutterudites, highlighting the influence of different filler atoms on these properties.

Findings

Light cation fillers are weakly coupled to the cage, resembling Einstein oscillators.

Bulk modulus varies from 77 GPa to 99 GPa depending on the filler.

Filler-cage bonding properties influence compressibility more than geometrical factors.

Abstract

Vibrational and elastic properties of the $R$Fe$_{4}$Sb$_{12}$ skutterudites are investigated by, respectively, temperature $(T)$ dependent extended X-ray absorption fine structure (EXAFS) and pressure $(P)$ dependent x-ray diffraction (XRD) experiments. The Fe $K$-edge EXAFS experiments of the $R=$ K, Ca and Ba materials were performed in the $T$-interval $6<T<300$ K and XRD experiments of the $R=$ Na, K, Ca, Sr and Ba materials were performed in the $P$-interval $1\text{ atm }<P<16$ GPa. From EXAFS, we obtained the correlated Debye-Waller parameters that were thus analyzed to extract effective spring constants connected with the Fe-$Y$ (where $Y=$ either $R$, Fe or Sb) scattering paths. Our findings suggest that in the case of the light cations, $R=$ K or Ca, the $R$ atoms are relatively weakly coupled to the cage, in a scenario reminiscent to the Einstein oscillators. From the XRD experiments, we obtained the bulk modulus $B_{0}$ for all $R=$Na, K, Ca, Sr and Ba materials, with values ranging from $77$ GPa ($R=$ K) to $R=99$ GPa ($R=$ Ba) as well as the compressibility $\beta$ as a function of $P$. The trend in $\beta$ as a function of the $R$ filler is discussed and it is shown that it does not correlate with simple geometrical considerations but rather with the filler-cage bonding properties.