Structure marcasite

Finally, many disulfides have a quite different structure motif, being composed of infinite three-dimensional networks of M and discrete Sj units. The predominate structural types are pyrites, FeSa (also for M = Mn, Co, Ni, Ru, Os), and marcasite (known only for FeS2 among the disulfides). Pyrites can be described as a distorted NaCl-type structure in which the rodshaped S2 units (S-S 217 pm) are centred on the Cl positions but are oriented so that they are inclined away from the cubic axes. The marcasite structure is a variant of the rutile structure (Ti02,... 

The pyrites and marcasite structures can be thought of as containing 82 units though the variability of the interatomic distance and other properties suggest substantial deviation from a purely ionic description. Numerous higher polysulfides S have been characterized, particularly for the more electropositive elements Na, K, Ba, etc. They are yellow at room temperature, turn dark red on being heated, and may be thought of as salts of the polysulfanes... 

MP2, MAs2 and MSb2 all have a compressed form of the marcasite structure, while the carbides MC have trigonal prismatic coordination in the WC structure. Several borides are known MB2 has nets of boron atoms. RunBg has branched chains while RU7B3 has isolated borons. 

The most prevalent modification of the disulfide, FeS2, is pyrite, which may be visualized as a distorted NaCl structure where the Fe atoms occupy sodium positions and S2 groups are placed with their centers at the chloride positions. Pyrite is a largely occurring crystal with semiconductor properties Eg = 0.95 eV). Another modification of FeS2 is the very similar to pyrite but somewhat less regular marcasite structure. 

Table III. Cl Shifts of Some Marcasite Structure Compounds...

Another class of sulfides of considerable importance are the disulfides, represented by FeS2, CoS2, and others. All these contain discrete S2 units with an S—S distance almost exactly equal to that to be expected for an S—S single bond. These assume one of two closely related structures. First there is the pyrite structure named after the polymorph of FeS2 that exhibits it. This structure may be visualized as a distorted NaCl structure. The Fe atoms occupy Na positions and the S2 groups are placed with their centers at the Cl positions but turned in such a way that they are not parallel to any of the cube axes. The marcasite structure is very similar but somewhat less regular. 

The marcasite structure is adopted by only one disulphide (FeS2) but also by a number of other chalconides and pnictides. Their structures fall into two groups, with quite differently proportioned (orthorhombic) unit cells, and there are apparently no intermediate cases the differences in bonding leading to the two types of marcasite structure are not yet understood. 

Compressed marcasite structure 0-55 0-48 CrSb2 FeP2 RuP2... 

P-P groups in compounds with the pyrites or marcasite structures, discrete P4 rings in IrP3 etc. (C0AS3 structure), chains in PdP2, layers in CdP4. 

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