What are intersitials in a crystal ?

Explanation

### Step-by-Step Solution:

1. **Understanding the Crystal Structure**:
- A crystal is composed of a regular arrangement of atoms or ions in a three-dimensional space. This arrangement can be visualized as a lattice structure.

2. **Identifying Vacant Sites**:
- Within this lattice, there are certain positions where atoms or ions can be located. Some of these positions may be unoccupied, which are referred to as vacant or unoccupied sites.

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