Question

Starting with a ccp array of anions, what structure types are generated by a) filling all tetrahedral sites with cations, b) filling half of the tetrahedral sites, e.g. T₊, with cations c) filling all octahedral sites with cations and d) filling alternate layers of octahedral sites with cations? Comment on the absence of any known structure type in one of these four categories.

Answer 1

a. filling all tetrahedral sites with cations

• Cations are present at all corners and the centre face of the cube.
• Anions occupy all tetrahedral sites.
• there are two tetrahedral sites per atom
• Each anion is surrounded by four cations
• eg: This is caaled fluorite structure for CaF2
• ratio 8:4, eg CaF2, ZrO2 formula MX2

b. filling half of the tetrahedral sites, e.g. T+, with cations in ccp

• ccp has anion is present at all corners of the cube and at the centre of each face
• no. of atoms each unit = 4
• 2 tetrahedral sites are present out of which half is occupied by T+ cation
• hence there is one cation for each anion.
• eg: ZnS, Cu2S

c) filling all octahedral sites with cations

• Anion is present at each corner and at the centre face of the cube
• there are 3 octahedral sites per atom
• cations (T+) occupies all octahedral sites
• there is one cation for one anion
• cation and anion coordination 6:6 or 1:1
• eg: NaCl, FeO

d. No such arrangement possible