Ceramics- Ionic Radii and Crystal Structure

Question 1

What energies contribute to the position of ions bonded together?

Answer 1

Coulomb energy of attraction between oppositely charged ions. This attracts them closer together as the energy term becomes more negative with decreasing separation. Repulsion energy due to quantum state. This becomes very positive at low separation.

Question 2

Graph of energy vs distance for inter atomic bonds

Answer 2

Very steep positive energy close to 0 distance. Down to a negative peak then curves back up to positive at large distance and stays nearly constant

Question 3

How does strength of ionic bond depend on valence?

Answer 3

Strength increases with valence so a bond between +3 and -3 is stronger than between +1 and -1

Question 4

What does an ion tend to do with neighbouring ions?

Answer 4

Tends to have the maximum number of neighbouring ions with opposite charge as possible

Question 5

Wha happens if the cation has a much lower radius than the anion?

Answer 5

The crystal structure would be unstable as the anions surrounding the cation would be too close together

Question 6

What is the coordination number determined by?

Answer 6

The ratio of the radii of the two ions

Question 7

Ionic radii ratios and coordination numbers

Answer 7

C/A>0: CN=2 linear
C/A>0.155: CN=3 triangle
C/A>0.225: CN=4 tetrahedron
C/A>0.414: CN=6 octahedron 
C/A>0.732: CN=8 cube

Question 8

Formula for strength of ionic bond donated from cation to anion

Answer 8

Charge of C/number of A around C

Strength of bond around cations should be equal to that around anions