Strongly decoupled europium and iron vibrational modes in filled skutterudites

The europium partial vibrational density of states (DOS) in $\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ and the iron partial vibrational DOS in both $\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ and ${\mathrm{Ce}}^{57}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ have been obtained by nuclear inelastic scattering. The results reveal the strong independence of the iron and rare-earth vibrational modes. The cage filling europium only participates significantly to the low-energy local vibrational modes. The force constants have been obtained from the measured probability of nuclear absorption. The energies of the peaks in the europium and iron DOS are in excellent agreement with the calculated DOS in $\mathrm{La}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$. The results indicate that nuclear inelastic scattering is the technique of choice for the study of the localized vibrational modes in thermoelectric “phonon glass” materials.

Figure 1

(a) The europium partial DOS in $\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ obtained at $25\mathrm{K}$ by europium-151 nuclear inelastic scattering. The solid line is a fit with three Gaussian peaks with energies of $7.3\pm 0.1$, $12.0\pm 0.4$, and $17.8\pm 0.5\mathrm{meV}$ and linewidths of $3.2\pm 0.2$, $4.4\pm 0.7$, and $4.4\pm 0.7\mathrm{meV}$. (b) The La projected vibrational DOS in $\mathrm{La}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$, calculated by Feldman et al. (Ref. 19). (c) The Fe projected vibrational DOS in $\mathrm{La}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$, calculated by Feldman et al. (Ref. 19). (d) The iron partial DOS in $\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ obtained at $295\mathrm{K}$ by iron-57 nuclear inelastic scattering. (e) The iron partial DOS in ${\mathrm{Ce}}^{57}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ obtained at $295\mathrm{K}$ by iron-57 nuclear inelastic scattering.

Figure 2

The Eu-151 nuclear inelastic scattering of $\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ observed at $295\mathrm{K}$ (open circles) and calculated at $295\mathrm{K}$ from the $25\text{−}\mathrm{K}$ DOS shown in Fig. 1a (closed circles). The spectra have been normalized on the peak at $7\mathrm{meV}$ and the experimental elastic peak at zero energy has been truncated.