Diamond structure elements .
C (diamond), Si, Ge, a-Sn (gray) .
Crystal lattice .
     The diamond crystal lattice structure can be modeled as an expanded FCC lattice .
where one-half of the tetrahedral cluster of close packed atoms have another equal .
radius atom of the same species located in the center of the cluster.  But the volume .


       Figure 52 - Expanded FCC
              diamond structure
of the central sphere is larger than the size of the interstitial void space normally
present in the center of a tetrahedral cluster of closest packed atoms.  This causes
the atoms located at the corners of the cluster to expand apart to accommodate the
central atom so that they no longer touch each other, only the central atom.  Each
atom of the diamond packing has a coordination number of four compared to the
twelve coordinated atoms of a normal FCC packing.
     In the following model of the diamond lattice one-half of the tetrahedra are
identified with an opaque color to indicate they contain a central atom (unoccupied
tetrahedral voids are blue).



click image to enlarge

  Figure 53 -  Diamond elements
                  lattice structure
Polyhedral framework
     The polyhedral framework of elements exhibiting the diamond structure can be
modeled as a array of vertex linked tetrahedra with the understanding that each
tetrahedron contains a central equi-radius atom.

linking surfaces



     Figure 54 -  Polyhedral framework
         model of the diamond structure

4 vertices

                         click image to enlarge

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