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1.5.1 Diamond Cubic Structure
- Typical and well known purely covalent bonded materials are carbon (Diamond), Si, Ge and SiC.
- For example, in diamond, the base lattice is FCC and is built by the C atoms with half of the tetrahedral sites filled by C atoms. Thus, the unit cell of diamond contains a total of 8 atoms.
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The structure is typically called as diamond cubic structure.
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Orbital hybridization of C atoms (sp3) requires that the atoms are tetrahedrally co-ordinated and thus the structure has high degree of directionality.
- One unit-cell consists of two FCC motifs, one at (0 0 0) and another at (¼ ¼ ¼).
- What it means is that there are two FCC unit-cells of C intermingled into each other, with origin of one at (0,0,0) and another at (¼,¼,¼).
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In case of compounds, FCC lattice can be formed by one type of atom and remaining atoms, usually from the same group, occupy half of the tetrahedral sites.
- Figure 1.21 shows the crystal structure of diamond where one can clearly observe the tetrahedrally co-ordinated C atoms.
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| Figure 1.21 Diamond cubic structure (a) the unit cell showing all the atoms and (b) (001)-plan view of the structure where positions marked show the position in the z-direction only while x- and y-positions are self-explanatory. |
- You can also work out the packing factor of this unit which is lower than the typical FCC unit cell. This is because the tetrahedral site size in a normal FCC unit cell is 0.225*r while in this structure, the size of the atom sitting at the site is much larger i.e. same size as the base lattice atom.(Self Evaluation)
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