It is an alternate form in which ZnS occurs in nature. The main features of this structure are
(i) Sulphide ions have HCP arrangement and zinc ions occupy tetrahedral voids.
(ii) Only half the alternate tetrahedral voids are occupied by Zn+2 ions.
(iii) Coordinate no. of Zn+2 ions as well as S−2 ions is 4. Thus, this structure has 4 : 4 coordination.
(iv) No. of Zn+2 ions per unit cell = 4(within the unit cell) ×1 + 6(at edge centres) × 1/3 = 6
No. of S−2 ions per unit cell = 12(at corners) × 1/6 + 2(at face centres) x 1/2 + 3(within the unit cell) x 1 = 6
Thus, there are 6 formula units per unit cell.
