Zinc Blende Structure: Arrangement of Ions in Unit Cell, Wurtzite Structure

Zinc sulphide (ZnS) is a chemical compound made up of zinc and sulphur. It exists as a white, crystalline powder that is often used as a pigment in paint, as well as an optical material in electronic devices such as LCDs and LEDs. Zinc sulphide is also used in the manufacturing of phosphors, the materials that emits light when exposed to an electric or optical stimulus. It has the ability to form two different crystalline structures or polymorphs; zinc blende and wurtzite. Zinc blende has been found thermodynamically more stable. 

This Chemistry article talks about zinc blende structure, arrangement of ions, wurtzite structure, calculation of the number of atoms present in one unit cell, and the packing fraction, along with the difference between the structure of zinc blende and diamond and some FAQs. 

Zinc Blende Structure 

Zinc sulphide is also known as zinc blende, which is the mineral form of the compound. It is a crystalline solid that has a hexagonal or trigonal crystal structure, depending on the specific variety. Zinc blende is typically a pale yellow or grayish color, and has a metallic luster. Zinc blende is a relatively common mineral and the primary ore of zinc. Historically, it was an important source of zinc, but today most of the world's zinc is produced from zincite, a zinc oxide mineral, which is more easily processed to extract the zinc. In addition to zinc, zinc blende often contain small amounts of cadmium, iron, and other elements. Zinc blende can be also doped with a variety of impurities to create different types of phosphors such as Panchromatic, Monochromatic, and so on.

In the crystal structure, the atoms are arranged in a cubic lattice (cubic close packing), with four atoms at each corner of the cube and one atom in the center of each face. The atoms in the corners are typical of one type, while the particles in the center of the faces are of a different type. 

This structure is cubic and symmetric due to the regular pattern of cations and anions, and this symmetry is also known as Isometric symmetry(cubic symmetry) in 3D. The unit cell of zinc blende crystal structure is a cube.

Arrangement of Ions in Zinc-Blende Unit Cell

The ions are structured in cubic close packing arrangement in the zinc blende structure. There are 4 octahedral voids and 8 tetrahedral voids in the CCP configuration.

• The  ions are arranged in a face-centered cubic structure (FCC). i.e., the  ions can be found in the cube's corner and in the center of each face.
• The alternate tetrahedral void contains the  ions.
• Four  ions surround each  ion tetrahedrally, while four  ions encircle each  ion tetrahedrally. Therefore, the structure has 4:4 coordination. 
• This creates a repeating pattern of cations and anions throughout the crystal, with the cations and anions being located in a face-centered cubic (FCC) lattice structure. 

Calculate the Number of Atoms Present in the Zinc Blende Unit Cell

In a unit cell of zinc blende, there are 4 zinc ions and 4 sulphur ions. Since each ion is considered to be one atom, the total number of atoms in the unit cell is the sum of the number of zinc atoms and the number of sulphur atoms, which is 4 + 4 = 8 atoms. The entire calculation can be expressed in the following steps: 

If we consider the alternate tetrahedral void space then the number of  ions present at the body center = 4

and the arrangement of sulfur ions in fcc manner indicates that,

Individual of 8  ions exists at the corner of the unit cell provides ⅛ or 8\times ⅛ = 1 

Following the same way if we see the individual of 6  ions present in the face center of the lattice it also contributes half or 6\times ½ = 3

consequently , it may be concluded from the above calculation is that the unit cell is composed of (1+3)=4  ions.

Calculate the Packing Fraction of the Zinc Blende Structure

The packing fraction of a crystal structure is a measure of how efficiently the atoms are packed together in the unit cell. It is defined as the ratio of the total atomic volume to the volume of the unit cell. In the zinc blende structure, the unit cell is a cube, and the atoms are located at the corners and center of each face. To calculate the packing fraction, we need to know the atomic radii of zinc and sulfur ions. The packing fraction of zinc-blende structure can be calculated by the following formula; 

Where, 

= packing fraction 

= radius of cations 

= radius of anions

Wurtzite Structure of Zinc Sulphide

Wurtzite is another crystal structure of zinc sulphide (ZnS). It is similar to zinc blende in that it is made up of zinc and sulphur ions, but the arrangement of the ions in the unit cell is different. In the wurtzite structure, the unit cell is hexagonal, with one zinc ion and one sulphur ion at the center of the hexagon. The remaining four zinc ions and four sulphur ions are located at the corners of the hexagon. 

Difference Between Zinc Blende and Diamond Structure

The key differences between zinc blende and diamond structure are listed as follows: 

Zinc Blende	Diamond Structure
The unit cell of zinc blende is a cube.	The unit cell of the diamond is a tetrahedron.
In zinc blende, there are 4 zinc ions at the corners and 4 sulphur ions at the center of each face of the cube-shaped unit cell.	In the diamond structure, each carbon atom is surrounded by four other carbon atoms at the corners of a tetrahedron.
Zinc blende is made up of zinc and sulphur ions.	Diamond is made up of carbon atoms.
It has a brown, yellow, or greenish appearance.	It exists as a brown, yellow, or colourless solid.
Zinc blende is a semiconductor material with moderate electrical conductivity.	Diamond is the hardest naturally occurring substance and has the highest thermal conductivity of any known material.

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