Siegenite

Siegenite was first described in 1850 for an occurrence in the Stahlberg Mine in Müsen, Siegerland, North Rhine-Westphalia, Germany and named for the locality.[2] It occurs in hydrothermal copper-nickel-iron sulfide bearing veins associated with chalcopyrite, pyrrhotite, galena, sphalerite, pyrite, millerite, gersdorffite and ullmannite.[3]

It occurs in a variety of deposits worldwide, including Brestovsko in the central Bosnian Mountains of Serbia; at Kladno in the Czech Republic; Blackcraig, Kirkcudbrightshire, Scotland. In the United States occurrences include the Mine la Motte of Madison County and the Buick mine, Bixby, Iron County and in the Sweetwater mine of Reynolds County in the Lead Belt of Missouri. In Canada, it is known from the Langis mine, Cobalt-Gowganda area, Ontario. In Africa it occurs at Shinkolobwe, Katanga Province and Kilembe, Uganda. In Japan, it is reported from the Kamaishi mine, Iwate Prefecture, and the Yokozuru mine, north Kyushu. It also occurs at Kalgoorlie, Western Australia.[3] It is found at the Browns deposit, Batchelor, Northern Territory, Australia.[2]

Conventional unit cell of NiCo₂S₄ looking down at the [100] direction. Gray atoms are Ni, blue atoms are Co, and yellow atoms are S.

Siegenite is a member of the thiospinel group, which belongs to the cubic space group ${\displaystyle Fd{\bar {3}}m}$ (#227) and has the Pearson symbol ${\displaystyle cF56}$. Similar to normal spinels, a normal thiospinel unit cell consists of eight FCC sub unit cells of two different types, where S^2- anions occupy all the FCC lattice points. The first type of sub unit cell has 2+ cations occupying 2 of the 8 tetrahedral sites and 3+ cations occupying 3/2 of the 4 octahedral sites. The second type of sub unit cell has only 3+ cations occupying 5/2 of the 4 octahedral sites. These two types of sub unit cells are alternatively stacked, forming a NaCl-type superstructure.

For a normal thiospinel (NiCo₂S₄), Ni²⁺ cations occupy 1/8 of the tetrahedral sites to form NiS₄ tetrahedra and Co³⁺ cations occupy 1/2 of the octahedral sites to form CoS₆ octahedra. Each tetrahedron shares corners with 12 neighboring octahedra, and each octahedron shares corners with 6 tetrahedra and edges with 6 octahedra. For an inverse thiospinel (CoNi₂S₄), Ni²⁺ occupy 1/8 of the octahedral sites and Co³⁺ occupy 1/4 of the tetrahedra sites and 1/4 of the octahedral sites. For a mixed/complex thiospinel, both metal ions occupy tetrahedral and octahedral sites and can be expressed as (A_xB_1-x)_Td[A_2-xB_x]_OhX₄ (0 < x < 1), where A and B are metal ions, x is the degree of inversion, and ${\displaystyle T_{d}}$ and ${\displaystyle O_{h}}$ denote the tetrahedral and octahedral sites, respectively.

The powder X-ray diffraction (XRD) pattern of siegenite exhibits strong diffraction signals between 20° and 60° 2θ angles. The lattice constant of siegenite is measured to be 9.319 Å based on the strongest reflection at around 32°, corresponding to lattice plane (311), which agrees with the calculated lattice constant of 9.325 Å.[9][10]

Unlike many binary and ternary semiconductor oxides, NiCo₂S₄ exhibits metallic properties and high electrical conductivity, which makes it useful as an electrode material in energy storage devices. The resistivity of NiCo₂S₄ is ~10³ μΩ cm at room temperature and its temperature coefficient of resistivity is positive and stays constant between 40 K and 300 K, which is indicative of a metallic compound.[9] NiCo₂S₄ also has a very low Seebeck coefficient of 5 μV K^–1 and a carrier density of 3.18 × 10²² cm⁻³ higher than that of silver.[9]

Reported synthetic routes of nickel cobalt sulfide include hydrothermal[11][12] and solvothermal[13] reactions, solvent-free thermal decomposition of xanthates,[14] SILAR method for thin films,[15] and solution-phase organometallic synthesis.[16] The hydrothermal reaction is the most widely used synthesis method to fabricate intricate nanostructures on highly porous substrates, yielding hierarchical structures that maximize redox-active surface areas and promote high-rate supercapacitive performance of Ni-Co-S-based electrodes.