In a hypothetical solid C atoms are found to from cubical close packed lattice. A atoms occupy all tetrahedral voids & B atoms occupy all octahedral voids. A and B atoms are of appropriate size, so that there is no distortion in CCP lattice of C atoms. Now if a plane as shown in the following figure is cut.Then the cross section of this plane will look like.
Seven crystal systems -
On the basis of edge length and interaxial angle,crystal system are of seven types- cubic, tetragonal, orthorhombic, monoclinic, hexagonal, triclinic, rhombohedral.
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From the sizes of octahedral and tetrahedral voids, it is clear that the atoms occupying these voids will not
touch each other as we have along body diagonal of FCC.