Siegenite

Siegenite
Siegenite from Buick mine, Bixby, Viburnum Trend District, Iron County, Missouri, USA
General
Category	Sulfide mineral Thiospinel group Spinel structural group
Formula (repeating unit)	(Ni,Co)3S4
IMA symbol	Seg
Strunz classification	2.DA.05
Crystal system	Cubic
Crystal class	m3m
Space group	Fd3m (#227)
Unit cell	a = 9.33 Å; V = 810.94Å3
Identification
Formula mass	304.3 - 305 g/mol
Color	Light to steel-grey, violet-gray (tarnished)
Crystal habit	As octahedral crystals, granular, massive
Twinning	On {111}; polysynthetic
Cleavage	Imperfect on {001}
Fracture	Irregular to uneven, sub-conchoidal
Mohs scalehardness	4.5 - 5.5
Luster	Metallic
Streak	Grayish black
Diaphaneity	Opaque
Density	4.5 - 4.8 g/cm3(Measured) 4.83 g/cm3(Calculated)
References

Siegenite(also called grimmite, or nickel cobalt sulfide) is a ternarytransition metaldichalcogenidecompound with the chemical formula (Ni,Co)₃S₄.It has been actively studied as a promising material system for electrodes in electrochemical energy applications due to its better conductivity, greater mechanical and thermal stability, and higher performance compared to metal oxides currently in use.Potential applications of this material system includesupercapacitors,batteries,electrocatalysis,dye-sensitized solar cells,photocatalysis,glucosesensors, andmicrowaveabsorption.

In synthetic chemistry, a range of chemical compositions with the formula Ni_xCo_3-xS₄(0 < x < 3) are often referred to as the siegenite system. However, according to the newIMAlist of minerals (updated November 2022), the normal spinel NiCo₂S₄is called grimmite, the inverse spinel CoNi₂S₄is called siegenite, and the endmembers Ni²⁺(Ni³⁺)₂S₄and Co²⁺(Co³⁺)₂S₄are calledpolydymiteandlinnaeite,respectively.In 2020, NiCo₂S₄(grimmite) is approved as a valid mineral species by the IMA.

Discovery and occurrence

Siegenite was first described in 1850 for an occurrence in the Stahlberg Mine inMüsen,Siegerland,North Rhine-Westphalia,Germanyand named for the locality.It occurs inhydrothermalcopper-nickel-iron sulfide bearingveinsassociated withchalcopyrite,pyrrhotite,galena,sphalerite,pyrite,millerite,gersdorffiteandullmannite.

It occurs in a variety of deposits worldwide, includingBrestovskoin the centralBosnian MountainsofSerbia;atKladnoin theCzech Republic;Blackcraig,Kirkcudbrightshire,Scotland.In the United States occurrences include theMine la MotteofMadison Countyand the Buick mine,Bixby,Iron Countyand in the Sweetwater mine ofReynolds Countyin theLead BeltofMissouri.In Canada, it is known from the Langis mine,Cobalt-Gowgandaarea,Ontario.In Africa it occurs atShinkolobwe,Katanga ProvinceandKilembe,Uganda.In Japan, it is reported from the Kamaishi mine,Iwate Prefecture,and the Yokozuru mine, northKyushu.It also occurs atKalgoorlie,Western Australia.It is found at the Browns deposit,Batchelor,Northern Territory,Australia.

Crystal structure

Conventional unit cell of NiCo₂S₄looking down at the [100] direction. Gray atoms are Ni, blue atoms are Co, and yellow atoms are S.

Siegenite is a member of thethiospinel group,which belongs to thecubicspace group$Fd{\bar {3}}m$(#227) and has thePearson symbol$cF56$.Similar to normalspinels,a normal thiospinel unit cell consists of eight FCC sub unit cells of two different types, where S^2-anions occupy all the FCClattice 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 aNaCl-typesuperstructure.

For a normal thiospinel (NiCo₂S₄), Ni²⁺cations occupy 1/8 of the tetrahedral sites to form NiS₄tetrahedra and Co³⁺cations occupy 1/2 of the octahedral sites to form CoS₆octahedra. 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$T_{d}$and$O_{h}$denote the tetrahedral and octahedral sites, respectively.

The powderX-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.

Electronic properties

Unlike many binary and ternary semiconductor oxides, NiCo₂S₄exhibits metallic properties and high electrical conductivity, which makes it useful as an electrode material in energy storage devices. Theresistivityof NiCo₂S₄is ~10³μΩ cm at room temperature and itstemperature coefficient of resistivityis positive and stays constant between 40 K and 300 K, which is indicative of a metallic compound.NiCo₂S₄also has a very lowSeebeck coefficientof 5 μV K^–1and a carrier density of 3.18 × 10²²cm⁻³higher than that of silver.

Synthesis

Reported synthetic routes of nickel cobalt sulfide includehydrothermalandsolvothermalreactions, solvent-free thermal decomposition ofxanthates,SILAR method for thin films,and solution-phaseorganometallicsynthesis.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.

Applications

Batteries and supercapacitors

(Ni,Co)₃S₄is a promising electrode material forbatteriesandsupercapacitors.Since theelectronegativityof sulfur is lower than that of oxygen, (Ni,Co)₃S₄has a more flexible lattice compared to its oxide counterpart, which allows easier electron and ion transport through the structure.Its high ionic conductivity can be attributed to the abundance of available cation sites in the thiospinel structure, and its high redox activity comes from the highly electrochemically active Ni²⁺/Ni³⁺and Co²⁺/Co³⁺redox couples. In literatures, nanoporous Ni-Co-S composite materials have been shown to have both high specific capacity in Li-based batteries and high capacitance in supercapacitors.

Electrocatalysis

(Ni,Co)₃S₄has been considered as an alternativeelectrocatalystforHERandOERreactions because of its high conductivity and low cost. It is reported that aoverpotentialof 87 mV for HER and 251 mV for HER can be achieved using NiCo₂S₄-based electrode, showing good potential forwater splittingapplications.