Skutterudite

Skutterudite is a cobalt arsenide mineral, often with nickel and iron replacing some of the cobalt. Its ideal chemical formula is CoAs3, though some sources note the arsenic subscript can range from 2 to 3.

Varieties high in nickel are called nickel-skutterudite, formerly known as chloanthite. This mineral forms in hydrothermal veins at moderate to high temperatures, alongside other nickel-cobalt minerals. It is mined primarily for cobalt and nickel, with arsenic recovered as a byproduct.

The mineral has a bright metallic luster and appears tin white or light steel gray, with a black streak. Its specific gravity is 6.5, and its hardness ranges from 5.5 to 6.

The crystal structure is isometric, forming cubes and octahedra similar to pyrite. When heated or crushed, the arsenic content produces a distinct garlic odor. Associated minerals include arsenopyrite, native silver, erythrite, annabergite, nickeline, cobaltite, silver sulfosalts, native bismuth, calcite, siderite, barite, and quartz.

Origins

Skutterudite has been known since the Middle Ages, used in the production of smalt. It was identified in 1845 at the Skuterud Mines in Modum, Buskerud, Norway. An alternative name for the mineral is smaltite. Notable occurrences include Cobalt, Ontario, and Franklin, New Jersey, in the United States.

Structure

The crystal structure of skutterudite was determined in 1928 by Oftedahl as cubic, belonging to space group Im 3 (number 204). The unit cell contains 32 atoms arranged in eight smaller cubes of cobalt atoms, which form octahedra with cobalt at their centers.

Six of these cubes are filled with planar square rings of arsenic, each oriented parallel to a unit cell edge. At the 2a Wyckoff position, there are two large structural voids, each about five angstroms in size, which can accommodate impurity atoms. This arrangement is known as the skutterudite structure.

Applications

Materials with this structure are studied as low-cost thermoelectric materials due to their low thermal conductivity. Synthesized skutterudites have achieved a thermoelectric figure of merit (ZT) close to 1 at 800 kelvins. A relatively high dimensionless figure of merit has been observed in polycrystalline skutterudite partially filled with ytterbium ions. The small but heavy ytterbium atoms occupy the voids in the CoSb3 host structure, lowering thermal conductivity while leaving the favorable electronic properties largely intact.