The Pyrite Structure

the alignment of the disulfide ions along the c-axis extends or elongates the unit cell in the c-direction and makes it tetragonal rather than isometric. 

This is a unit cell that has two angles that are not 90°. (Remember that the isometric and tetragonal unit cells have 90° angles, and the hexagonal unit cell has a 120° angle and a three-fold axis.) 

There are three perpendicular two-fold rotational axes. There are also mirror planes that contain each of the axes, and a center of symmetry. 

Based upon your identification of symmetry elements, give the crystal system and the point group designation. 

Orthorhombic, 2/m2/m2/m. The orthorhombic system contains at least one twofold axis and vertical mirror planes or perpendicular two-fold axes. Notice that it is not necessary for all of the angles between the axes to be 90° in order to have an orthorhombic crystal. The symmetry elements are the necessary determinants of the classification. 

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The sulfides are fewer and less familiar than the oxides but, as is to be expected, favour lower oxidation states of the metals. Thus manganese forms MnS2 which has the pyrite structure (p. 680) with discrete Mn and 82 ions and is converted on heating to MnS and... 

Ruthenium and osmium form only disulfides. These have the pyrite structure and are diamagnetic semiconductors this implies that they contain M . RuSc2, RuTc2, OsSc2 and OsTc2 are very similar. All 6 dichalcogenides are obtained directly from the elements. 

The mineral laurite is the mixed sulphide (Ru,Os)S2 this and RuS2 and OsS2 have the pyrite structure as does RuQ2 (Q = Se, Te). These can be made from the reaction of the chalcogen with the metals, while RuCl3 will also react with Se and Te. 

The pyrites structure is exhibited by several pnictides MAs2 and MSb2 (M = Pd, Pt) and PtP2 (Figure 3.10). 

The oxidation state of the metals in the bulk sulfides corresponds to what is expected for a given metal in a sulfur environment. Generally the lattice sulfur is tightly bound to several metal atoms (usually three) and is very stable. Consequently, narrow valence distributions are observed. For the early layered TMS, the oxidation state is 4+. In the later isotropic TMS, the valence is usually 2+ or 4+. For example, in RuS2 the oxidation state is 2+ because of the pyrite structure. In Rh2S3 the oxidation state is 3 +. In other TMS inthe stable catalytic state such as amorphous OsS, it is not clear what the oxidation state is although formally it would be 2+. 

From similar data for other crystals with the pyrite structure or a closely related structure (of the marcasite or arsenopyrite types), given... 

It fits into this picture that the normal marcasites easily transform under pressure to the pyrite structure whereas the loellingites FeP2 and FeAs2 as well as the arsenopyrite-type representative C0AS2 retain their structure up to at least 65 kbar/400- 1100° C (16). 

Meanwhile, the structure of Rl Ses has been determined by Hohnke and Parthe (491). The space group of this structure is R3—C31. The cations are located at positions 3(e), the empty cation site of the pyrite structure being 1 (a) 000. Thus, in a layer of cations, which in the pyrite structure are in a close-packed arrangement, every second row is only... 

Another unexpected example of non-metallic d6 phases is offered by this group of compounds. Since RhSeD(h) crystallizes in the pyrite structure... 

When the distortion of the anion octahedra in PdS2 is reduced by pressure the semiconductor transforms into a metal before a change to the pyrite structure takes place (16). 

Whereas palladium is divalent in the non-metallic PdP2, the pyrite structure of the diarsenide and diantimonide suggests a Pd valency of four, although these compounds show metallic properties. The metallic behaviour of the bismuthide is not surprising since the metallic character increases with heavier anions. PdBi2 crystallizes in two modifications... 

It will not be possible, in this paper, to deal with all of the platinum metal chalcogenides. Instead, a number of examples will be chosen and their electrical as well as magnetic properties correlated with the atomic positions in the various structures formed. The first group of compounds to be discussed crystallize with the pyrite structure, which is shown in Figure 1. This structure is similar to the NaCl structure if we replace Na by Fe and each Cl by an S2 group. However, the S-S distance within... 

Table I. Platinum Metal Compounds with the Pyrite Structure...

The metallic properties observed for the d7 compounds listed in Table I are also consistent with the Goodenough model. The rhodium-selenium system is of particular interest and demonstrates clearly the important relationships between structure and transport properties. Cations may be removed from the superconducting compound (Tc = 6°K), RhSe2. The pyrite structure is maintained as the eg band is gradually emptied, and at the composition Rh2/3Se2 (RhSe3), all cations are trivalent—i.e., have the configuration 4d6. It is not known yet if the ideal... 

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 characteristic structure parameters of the pyrite structure, as obtained by the DFT calculations, are given in Tab.l in comparison to the experimental values. For the measured lattice parameters a, FeS2 has a minimum. The position parameters increase... 

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