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Tetrahedral void

A tetrahedral void is formed when an atom fits into the depression formed by three other closest packed atoms (forming an equilateral triangle).
 
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In general, the number of effective tetrahedral voids in a unit cell is double the number of effective atoms in that unit cell. Let the radius of host atom (forming fcc lattice) be rh, while the radius of foreign atom entering a tetrahedral void in an ideal fcc lattice be rf, then the sum of radii of host and foreign atoms in terms of edge length (a) is given by
 
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Octahedral void

An octahedral void is formed when three closest packed atoms of one layer (forming an equilateral triangle) is placed over three closest packed atoms of the second layer, their positions being inverted with respect to each other. Each atom touches four other atoms, except the atoms diagonally opposite to it.
 
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The effective number of octahedral voids in a unit cell is equal to the number of effective atoms in that unit cell.
 
Let the radius of host atom (forming fcc lattice) be rh and the radius of foreign atom occupying octahedral void in an ideal fcc lattice be r f, then the distance between a host atom and a foreign atom is given by
 
Description: 25712.png
 
where a is the edge length of an ideal fcc lattice.
 
Description: 27937.png




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