Effect of temperature on crystal structure

Increase of temperature decreases the coordination of number, e.g. upon heating to 760 K, the CsCl type crystal structure having coordination 8:8 changed to NaCl type crystal structures having coordination 6:6.

                               hightemp

CsCl type crystal  ——————> NaCl type crystal

(8.8 coordination)                        (6.6 Co-ordination)

Effect of pressure on crystal structure

Increase of pressure increases the Co – ordination number during crystallization e.g. by applying pressure, the NaCl type crystal structure having 6:6 coordination number changes to CsCl type crystal having coordination number 8:8

                             highpressure

NaCl type crystal  ——————> CsCl type crystal 

(8.8 coordination)                        (6.6 Co-ordination)

Illustration 39. Out of NaCl and CsCl, which one is more stable and why?

Solution:          CsCl is more stable than NaCl. This is because higher the coordination number, greater are the forces of attraction between the cations and the anions in the close-packed arrangement. As CsCl has co-ordination number of 8 : 8 while NaCl has a coordination number of 6 : 6, therefore CsCl is more stable.

Illustration 40. Compare the structure of zinc blend (ZnS) with that of diamond.

Solution:          The structure of zinc blend (ZnS) is similar to that of diamond. In diamond each carbon atom is linked to four other atoms tetrahedrally. Similarly, in zinc blende, each Zn⁺ ion is surrounded by 4s^2- ions tetrahedrally and each S^2- ion is surrounded by 4Zn²⁺ ions tetrahedrally. Thus if all Zn²⁺ ions and S^2- ions in zinc blende are replaced by carbon atoms, if gives rise to structure of diamond.

Illustration 41.  Match List - X with List - Y

                        (A) AB, BC, CD, DA                              (B) AB, BD, CA, DC

                        (C) AD, BA, CB, DC                              (D) AC, BD, CB, DA

Solution:          (B)

Exercise 12.

MgO has a structure of NaCl and TlCl has the structure of CsCl. What are the coordination numbers of ions in each?