Skutterudites, filled

Figure 7.6. A filled. skutterudite antimonide crystal structure. A transition niclal atom (Fc or Co) at the centre of each octahedron is bonded to antimony atoms at each corner. The rare earth atoms (small spheres) are located in cages made by eight octahedra. The large thermal motion of rattling of the rare earth atoms in their cages is believed be responsible for the strikingly low thermal conductivity of these materials (Sales 1997).

The transition-metal monopnictides MPn with the MnP-type structure discussed above contain strong M-M and weak Pn-Pn bonds. Compounds richer in Pn can also be examined by XPS, such as the binary skutterudites MPn , (M = Co, Rh, Ir Pn = P, As, Sb), which contain strong Pn-Pn bonds but no M-M bonds [79,80], The cubic crystal structure consists of a network of comer-sharing M-centred octa-hedra, which are tilted to form nearly square Pnn rings creating large dodecahedral voids [81]. These voids can be filled with rare-earth atoms to form ternary variants REM Pnn (RE = rare earth M = Fe, Ru, Os Pn = P, As, Sb) (Fig. 26) [81,82], the antimonides being of interest as thermoelectric materials [83]. 

Fig. 26 Skutterudite-type structure in terms of a framework of M-centred octahedra and b cubic arrangement of M atoms with Pn4 rings and dodecahedral cages filled with RE atoms in the ternary variants. Reprinted with permission from [110]. Copyright the American Chemical Society...

Ce filled skutterudites 12 14.3. Future of filled skutterudites as thermo- ... 

Fig. 2. Positional parameters for the pnicogen atoms of filled and unfilled skutterudites (figure from Kaiser and Jeitschko (1999)). The straight line indicates the condition for square pnicogen rings. In the figure T = Co, Rh or Ir A = light lanthanides or Yb, Ca, Sr or Ba R = light lanthanides.

Fig. 3. Model of the filled skutterudite structure. The transition metal atoms (Fe, Ru, or Os - small light blue...

Fig. 4. Model of filled skutterudite structure that emphasizes the pnicogen-pnicogen bonding that results in nearly square pnicogen rings. The transition metal atoms (small white spheres) form a simple cubic lattice as shown. The lines connecting the transition metal atoms have been added for clarity and do not correspond to a chemical bond. The only chemical bonds shown in this model are those that form the pnicogen rings. The lanthanide atoms (large dark spheres) occupy the two voids without a pnicogen ring.

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