What are interstitials in a crystal ?

Explanation

**Step-by-Step Solution:**

1. **Definition of Interstitials**: Interstitials in a crystal refer to the atoms or ions that occupy the vacant spaces (voids) within the crystal lattice structure.

**Hint**: Think of interstitials as "guests" that fit into the "empty rooms" of a crystal.

2. **Understanding Crystal Structure**: Crystals are made up of a regular arrangement of atoms or ions that are closely packed together. However, within this packing, there are small spaces or voids.

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