Question

Several different structures are observed for metal oxides, MO2

Based on your understanding of what factors influence unit cell structure. discuss the following observation

a)Hf2, CeO2, and ThO2 exhibit the fluorite lattice type

b)SnO2 and PbO2 exhibit the rutile tyre structure

Si has different arrangement referred to as silica (tetrahedral arrangement similar to diamond)

Answer 1

a) As we know, in fluorite structure, the metal ion forms the fcc unit cell and anion is present tetrahedral voids. This is feasible only when anions are small enough to fit into the tetrahedral voids. In the given examples, the metal ions are much bigger in size than the oxide ion. Thus, it is the cation, not the anion that forms the fcc.

b) The size ratios of cations and anions ensures that Sn⁴⁺ and Pb⁴⁺ are able to fit into the octahedral holes of the anion fcc. Thus, it assumes rutile structure.