1. Frenkel defect is favoured by the small size of cations. In alkali metal halides both cations and anions are of almost same size.
2. Number of atoms of P = \(\frac{1}{8}\) × 8 = 1
Number of atoms of Q = \(\frac{1}{4}\) × 12 = 3
Number of atoms of R = 1
∴ Formula of the compound is PQ3R