Class: Rare Mineral, Intermetallic
Related Structures: Perovskite, Rhenium Trioxide, Layered Perovskite
Notable Properties: Thermoelectric, Reactive Subunit
Skudderudite or cobalt triasenide is a natural occurring mineral often found with small quantities of nickel and iron incorporated into the structure. The crystal structure is cubic with space group Im-3 and can be described as a highly distorted perovskite structure type. Like perovskite, the skutterudite structure contains void spaces that can be filled by large cations (lanthanides, actinides, thallium, barium…) to yield “filled skutterudites” with the general formula RE1Co4As12; these, however, are not naturally occuring. Unlike, perovskites, the distorted lattice limits results in only 1 in 4 of the voids spaces open enough for extra framework cation inclusion.
The other 3 in 4 voids actually have As-As contacts close enough to be considered formal bonds. As such in addition to the structure analogous to the rhenium trioxide topology the structure can also be draw as distance As44– polyanions stabilized in a cobalt matrix. This perspective better describes the chemical bonding in the structure by yielding reasonable oxidation states for both Co and As. Phase pure binaries with this structure type can be made with Co, Rh, and Ir metal centers and P, As, and Sb polyanions.
The synthetic and off stochiometric skutterudites are best known for their phenomenal thermoelectric properties. Figures of merit, ZT, as high at 1.25 at 900 K for Ba0.3Ni0.05Co3.95Sb12 have been observed. The rational for the exceptional thermoelectric performance of the filled skutterudites stems from the phonon glass electron crystal model of an ideal thermoelectric material, and the imagining of the extra framework cations “rattling” with in a cage to yield very low thermal conductivities.