Crystallography
Iron-pyrite FeS2 represents the prototype compound of the crystallographic pyrite structure. The structure is simple cubic and was among the first crystal structures solved by X-ray diffraction. It belongs to the crystallographic space group Pa3 and is denoted by the Strukturbericht notation C2. Under thermodynamic standard conditions the lattice constant of stoichiometric iron pyrite FeS2 amounts to 541.87 pm. The unit cell is composed of a Fe face-centered cubic sublattice into which the S ions are embedded. The pyrite structure is also taken by other compounds MX2 of transition metals M and chalcogens X = O, S, Se and Te. Also certain dipnictides with X standing for P, As and Sb etc. are known to adopt the pyrite structure.
In the first bonding sphere, the Fe atoms are surrounded by six S nearest neighbours, in a distorted octahedral arrangement. The material is a diamagnetic semiconductor and the Fe ions should be considered to be in a low spin divalent state (as shown by Mössbauer spectroscopy as well as XPS), rather than a tetravalent state as the stoichiometry would suggest.
The positions of X ions in the pyrite structure may be derived from the fluorite structure, starting from a hypothetical Fe2+(S-)2 structure. Whereas F- ions in CaF2 occupy the centre positions of the eight subcubes of the cubic unit cell (¼ ¼ ¼) etc., the S- ions in FeS2 are shifted from these high symmetry positions along <111> axes to reside on (uuu) and symmetry-equivalent positions. Here, the parameter u should be regarded as a free atomic parameter that takes different values in different pyrite-structure compounds (iron pyrite FeS2: u(S) = 0.385 ). The shift from fluorite u=0.25 to pyrite u=0.385 is rather large and creates a S-S distance that is clearly a binding one. This is not surprising as in contrast to F- an ion S- is not a closed shell species. It is isoelectronic with a chlorine atom, also undergoing pairing to form Cl2 molecules. Both low spin Fe2+ and the disulfide S22- moeties are closed shell entities, explaining the diamagnetic en semiconducting properties.
The S atoms have bonds with three Fe and one other S atom. The site symmetry at Fe and S positions is accounted for by point symmetry groups C3i and C3, respectively. The missing center of inversion at S lattice sites has important consequences for the crystallographic and physical properties of iron pyrite. These consequences derive from the crystal electric field active at the sulfur lattice site, which causes a polarisation of S ions in the pyrite lattice. The polarisation can be calculated on the basis of higher-order Madelung constants and has to be included in the calculation of the lattice energy by using a generalised Born-Haber cycle. This reflects the fact that the covalent bond in the sulfur pair is ill accounted for in the strictly ionic treatment of Madelung theory.
Arsenopyrite has a related structure with heteroatomic As-S pairs rather than homoatomic ones. Marcasite also possesses homoatomic anion pairs, but the arrangement of the metal and diatomic anions is different than in a pyrite. Despite its name a chalcopyrite does not contain dianion pairs, but single S2- sulfide anions.
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