Question

A crystal is made up of particles A and B. A forms FCC packing and B occupies all the octahedral voids. If all the particles along one of the diagonal plane in the unit cell are removed, then, the formula of the crystal would be :

(a) AB (b) A₅B₇ (c) A₇B_{5 (d) none of these}

Answer 1

A forms FCC lattice. So the number of atoms =4
B atoms are in octahedral voids. Number of octahedral voids = Number of atoms = 4
We are removing voids from diagonal of body (diagonal plane). But octahedral voids are in body centre and at edge centre. Tetrahedral voids only will be present in diagonal. So removing voids from diagonal means removing tetrahedral voids. Octahedral voids will not be affected.
So the formula of the compound is A4B4 or AB