In a unit cell of hexagonal closed packing structure, there are total six number of oxide ions per unit cell. This means there are 6 octahedral voids are present. The number of octahedral voids occupied by ferric ions can be calculated as:
2
3
×6=4 The ratio of metal and ion is 4 : 6 = 2 : 3 . Therefore, the correct formula of metal oxide is M2O3