A tetrahedral void is basically the empty space that is found in substances having tetrahedral crystal systems. In general, voids are described as empty spaces that are present in crystal systems. These voids arise mainly because of the various arrangements of atoms. So in the case of tetrahedral voids, these are present among four spheres that have a tetrahedral arrangement. Usually, there are two main types of voids. One is the tetrahedral void, as mentioned earlier, and the other one is called an octahedral void, which is mostly seen in substances having octahedral crystal systems.
In any case, we will learn more about tetrahedral voids here.
Tetrahedral Void Diagram
If we look at the diagram below, there are two types of three-dimensional close packing in crystals.
One type is known as cubic close packing. In this case, the two-dimensional structures are stacked in a certain or specific alignment. We will further see that the layers alternate with each other. As a result, the second layer is found, or it occurs in the depression of the first layer.
Also Read: Crystal Lattices and Unit Cells
A triangle-shaped void is observed in this type of alignment. The sphere which is found in the depression will form a void between itself and another sphere that is in the layer above. This void that is formed is basically what we call the tetrahedral void. In simple words, when a sphere of the second layer is above the void of the first layer, a tetrahedral void is formed. It is marked in “T” in the diagram.
Tetrahedral Void Video Lesson
Characteristics of Tetrahedral Void
Here is a summary of the characteristics of a tetrahedral void.
- A single triangular void or vacant space in a crystal is surrounded by four (4) atomic spheres. Therefore, the coordination number of the tetrahedral void is also 4.
- In a tetrahedral void, the atom comes in contact with four atoms that are placed at four corners of a tetrahedron.
- This void is formed when a sphere of the second layer is above the void of the first layer.
- The volume of the void is considerably smaller than that of the spherical particle.
Calculation of Tetrahedral Void in a Lattice
Usually, the number of voids depends on the number of close-packed spheres. That being said, there is a simple way to calculate the number of tetrahedral voids in a lattice.
In this case, we will say if the number of close-packed spheres (i.e., unit cells) is said to be “n”, then the number of voids will be twice as many. Thus, the number of tetrahedral voids will be “2n”.
On the other hand, if R is the radius of the constituent spherical particle, then the radius of the tetrahedral void will be 0.225 R.
The radius of a tetrahedral void r / R = 0.225
It is to be noted that the radius of the sphere that is accommodated in an octahedral hole without disturbing the structure should not exceed 0.414 times that of the structure forming a sphere.
Also Read: Crystal Structure
Differences between Tetrahedral and Octahedral Void
Let us see some differences between the tetrahedral and octahedral voids.
| Tetrahedral void | Octahedral void |
|---|---|
| Present in substances having a tetrahedral crystal system. | Present in substances having an octahedral crystal system. |
| A single triangular void in a crystal is surrounded by four (4) spheres. | A double triangular void like c is surrounded by six (6) spheres. |
| Observed in the edges of the unit cell. | Observed in the centre of the unit cell. |
| The coordination number of the tetrahedral void is 4. | The coordination number of the tetrahedral void is 6. |
| Two tetrahedral voids per sphere. | Two octahedral voids per sphere in the crystal lattice |
| Tetrahedral voids have a bigger volume. | Comparatively smaller volume than tetrahedral voids. |
Octahedral Void Video Lesson
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