Zinc Blende Structure

What is the Zinc Blende structure?

Zinc Blende is the name given to the mineral Zinc Sulphide. Zinc Sulphide is an inorganic compound with the chemical formula of ZnS. 

Zinc Sulphide is an ionic compound of AB type, i.e it has a ratio of 1:1 between cation and anions. Zinc sulphide has two crystalline forms namely. Zinc Blende and Wurtzite. The more stable cubic form is known as zinc blende.  The hexagonal form is known as the mineral Wurtzite, although it also can be produced synthetically. In both the structures, stoichiometry is maintained i.e, 1:1 ratio between Zn and S atoms. The difference between these two polymorphs arises due to the different arrangement of ions in their structure.

Table of Contents

• Structure of zinc blende
• Arrangement of ions in zinc blende unit cell
• Calculate the number atoms present in zinc blende unit cell
• Calculate the packing fraction of zinc blende structure
• Wurtzite structure of Zinc Sulphide
• Frequently Asked Questions

Structure of Zinc Blende

Arrangement of ions in zinc blende unit cell

• In the Zinc Blende structure, the ions are arranged in CCP(Cubic Close Packing) manner. In the CCP arrangement, there are 8 tetrahedral voids and 4 octahedral voids present.
• The S^2- ions are present in FCC (Face Centered Cubic) arrangement. i.e The S^2-  ions are present at the corner of the cube and at the centre of each face.
• The Zn²⁺ ions are present in the alternative tetrahedral void. 
• Each Zn²⁺ ion is surrounded tetrahedrally by four S^2- ions and each S^2- ion is surrounded tetrahedrally by four Zn²⁺ ions. Thus the structure has 4:4 coordination.
• The radius ratio of ZnS is  r_Zn²⁺ /r_S2- =  74/184 = 0.4

Calculate the number of atoms present in the zinc blende unit cell

In the unit cell of Zinc blend (ZnS) structure, numbers of Zn²⁺ and S^2- ions are calculated as:

(a) Number of Zn²⁺ ions present in alternative tetrahedral voids = 4 (present at the body centres) =4

(b) Sulphide (S^2-) present in fcc manner, 

8 S^2– ions present at the corners of the unit cell contribute ⅛ each = 8 × ⅛ = 1

6 S^2- ions present at the centre of the face contribute ½ each = 6 × ½  = 3

So the total number of S^2- ions present in the unit cell = 1+3 =4

Calculate the packing fraction of zinc blende structure

The packing fraction of the zinc blende structure can be calculated as follows

In the fcc unit cell,  4r = ✓2 a

Packing fraction (Φ) :

Wurtzite structure of Zinc Sulphide

• This structure is different from the Zinc Blende Structure.
• In the Wurtzite structure, the sulphide ion is arranged in the hcp (hexagonal close-packed) structure. 
• The Zn²⁺ ions occupy half of the tetrahedral voids.
• This structure has a 4: 4 coordination number.
• No. of Zn⁺² ions per unit cell = {4 (within the unit cell) × 1} + {6 (at edge centres) × 1/3} = 6
• No. of S^-2 ions per unit cell = {12 (at corners) ×1/6} + {2 (at face centres) × ½ } + {3 (within the unit cell) ×1} = 6
• Thus, there are 6 formula units per unit cell.