Polynary Diamond-like Compounds

Of the numerous ternary and polynary diamond-like compounds we deal only with those that can be considered as superstructures of zinc blende. A superstructure is a structure that, while having the same structural principle, has an enlarged unit cell. When the unit cell of zinc blende is doubled in one direction (c axis), different kinds of atoms can occupy the doubled number of atomic positions. All the structure types listed in Fig. 12.8 have the tetrahedral coordination of all atoms in common, except for the variants with certain vacant positions. 

CuFeS2 (chalcopyrite) is one of the most important copper minerals. Red /3-Cu2Hgl4 and yellow j3-Ag2Hgl4 (CdGa2S4 type) are thermochromic they transform at 70 °C and 51 °C, respectively, to modifications having different colors (black and orange) in these the atoms and the vacancies have a disordered distribution. 

Aside from the superstructures mentioned, other superstructures with other enlargement factors for the unit cell are known, as well as superstructures of wurtzite. Defect structures. 

Superstructures of the zinc blende type with doubled c axis 

The numbers next to the circles designate the height in the direction of view 

Hexagonal-primitive packing of Si-V one unit cell is outlined. Si-VI only atomic contacts within the layers parallel to the b-c plane have been drawn Numbers x coordinates 

Germanium forms the same kinds of modifications as silicon at similar conditions (Fig. 12.4). Tin, however, does not exhibit this diversity )3-tin transforms to a body-centered cubic packing of spheres at 45 GPa. Lead already adopts a cubic closest-packing of spheres at ambient pressure. 

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