Transition metal $d$-band occupancy in skutterudites studied by electron energy-loss spectroscopy

The transition-metal $3d$ occupancy of a series of thermoelectric skutterudites is investigated using electron energy-loss spectroscopy. We find that bonding causes an emptying of the $3d$ states in the binary skutterudites $\mathrm{Co}{\mathrm{P}}_3$, $\mathrm{Co}{\mathrm{As}}_3$, $\mathrm{Co}{\mathrm{Sb}}_3$, and $\mathrm{Ni}{\mathrm{P}}_3$, while compared to the pure Fe the $3d$ occupancy in $\mathrm{La}{\mathrm{Fe}}_4{\mathrm{P}}_{12}$ is significantly increased, consistent with the idea that each interstitial La atom (rattler) donates three electrons to compensate for missing valence electron of Fe as compared to Co. These experimental results are in agreement with previous models suggesting a predominantly covalent bonding between transition metal and pnictogen atoms in skutterudites, and provide evidence of charge transfer from La to the Fe-P complex in $\mathrm{La}{\mathrm{Fe}}_4{\mathrm{P}}_{12}$.

Figure 1

Example of how a fitted power-law model was used to remove the background contribution at the Fe $L_{2,3}$ edge.

Figure 2

Normalized EEL spectra of the Co $L_{2,3}$ edge of pure cobalt and the three Co-based skutterudites.

Figure 3

Normalized spectra from pure nickel and $\mathrm{Ni}{\mathrm{P}}_3$.

Figure 4

Normalized spectra from the pure iron and $\mathrm{La}{\mathrm{Fe}}_4{\mathrm{P}}_{12}$ samples. The sharp absorption peaks at high energy side are from the La $M_{4,5}$ transitions.