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 4s2- ions tetrahedrally and each S2- ion is surrounded by 4Zn2+ ions tetrahedrally. Thus if all Zn2+ ions and S2- ions in zinc blende are replaced by carbon atoms, if gives rise to structure of diamond.  

Illustration 41.  Match List - X with List - Y  

 

LIST - X

LIST - Y

(A)

Zinc blende structure

(A) oxide ions in hcp and 2/3rd of octahedral voids occupied by trivalent cation.

(B)

Anti-fluorite structure

(B) anion in FCC arrangement and alternate tetrahedral voids occupied by bivalent cation.

(C)

Corundum structure

(C) Oxide ions in CCP, bivalent cations in corners and one tetravalent cation in the octahedral void created by oxide ions.

(D)

Pervoskite structure

(D) Anions in FCC and cations in all tetrahedral voids

                          (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?

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