The regular three dimensional arrangement of points in a crystal is known as crystal lattice and the smallest repeating pattern in the lattice is called unit cell. The unit cells are characterised by the edge lengths a, b, c and the angles between them `alpha, beta and gamma` respectively. Based on this, there are seven crystal systems. In a cubic unit cell:
`a=b=c and alpha = beta=gamma=90^(@)` The number of points in simple, body centred and face centred cubic cells are 1,2 and 4 respectively In both the hcp and ccp of spheres, the number of tetrahedral voids per sphere is two while the octahedral voids is one.
A double triangular void surrounded by three spheres above and three spheres below is called

Options

A. triangular void
B. tetrahedral void
C. octahedral void
D. trigonal bipyramidal void

Explanation

To solve the question regarding the term for a double triangular void surrounded by three spheres above and three spheres below, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Crystal Lattice and Unit Cell**:
- A crystal lattice is a three-dimensional arrangement of points (atoms, ions, or molecules) in a crystal.
- The smallest repeating unit of this lattice is called a unit cell, characterized by its edge lengths (a, b, c) and angles (α, β, γ).

2. **Identifying Types of Unit Cells**:
- There are different types of cubic unit cells: simple cubic (1 point), body-centered cubic (2 points), and face-centered cubic (4 points).
- In cubic unit cells, all edges are equal (a = b = c) and all angles are 90 degrees (α = β = γ).

3. **Understanding Voids in Close-Packed Structures**:
- In close-packed structures like hexagonal close-packed (hcp) and cubic close-packed (ccp), voids can occur between the spheres.
- Tetrahedral voids and octahedral voids are types of interstitial spaces formed by the arrangement of spheres.

4. **Identifying the Double Triangular Void**:
- A double triangular void is formed when there are three spheres above and three spheres below a central void.
- This arrangement corresponds to the octahedral void, which is surrounded by six spheres.

5. **Conclusion**:
- Based on the description of the void being surrounded by three spheres above and three spheres below, the correct term for this void is "octahedral void".

### Final Answer:
The double triangular void surrounded by three spheres above and three spheres below is called an **octahedral void**.

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