In mixed oxide , oxide ions form FCC lattice, cations M occupy 2/3rd od octahedral void and cation N occupy 1/3rd of tetrahedral void. Obtain the formula of compound.
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Solution
Oxide ions forms FCC lattice.
N occupy 1/3rd of tetrahedral voids.
M occupy 2/3rd of octahedral voids.
The number of tetrahedral voids formed is equal to twice the number of oxide ions forming FCC lattice and only 1/3rd of these are occupied by the N cation while ​The number of octahedral voids formed is equal to the number of oxide ions forming FCC lattice and only 2/3rd of these are occupied by the M cation.
Thus, the ratio of the number of atoms of O , N and M will be
1: 21/3 : 2/3 or 3:2:2 and the formula of the compound would be N2M2O3.