`NiO` is antiferromagnetic. But on heating at `250°C`, it becomes paramagnetic. Why?

Explanation

To understand why NiO is antiferromagnetic at room temperature but becomes paramagnetic upon heating to 250°C, we can break down the explanation into several steps:

### Step 1: Understanding Antiferromagnetism
- **Definition**: In antiferromagnetic materials, the magnetic moments of atoms or ions align in opposite directions, resulting in a net magnetic moment of zero.
- **NiO Structure**: In solid NiO, the nickel ions (Ni²⁺) and oxide ions (O²⁻) are arranged in a lattice structure. The magnetic moments of Ni²⁺ ions are aligned antiparallel to each other.

### Step 2: Heating the Material
- **Effect of Temperature**: When NiO is heated to 250°C, the thermal energy increases the vibrational motion of the ions in the lattice.
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