Formula of a Compound and Number of Voids Filled
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The number of octahedral voids present in a lattice = The number of close packed particles.
o The number of tetrahedral voids produced is twice this number.
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In ionic solids, the bigger ions i. e. anions forms close packed structure and the smaller ions i. e. cations occupy the voids.
o If the latter ion is small then tetrahedral voids are occupied, if bigger, then octahedral voids are occupied.
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In a compound, the fraction of octahedral or tetrahedral voids that are occupied depends upon the chemical formula of the compound.
o Close packed structures have both tetrahedral and octahedral voids.
Locating Tetrahedral Voids
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Take a unit cell of ccp or fcc lattice divide into eight small cubes with each small cube having atoms at alternate corners.
Thus, each small cube has 4 atoms that on joining to each other make a regular tetrahedron.
Thus, there is one tetrahedral void in each small cube and eight tetrahedral voids in total.
Each of the eight small cubes have one void in one unit cell of ccp ccp structure has 4 atoms per unit cell. Thus, the number of tetrahedral voids is twice the number of atoms.