`NiO` is antiferromagnetic. But on heating at `250^(@)C`, it becomes paramagnetic. Why?

Explanation

In solid state, the magnetic moments of Nio are aligned in such a fashion, such that the net moment is zero. `So, NiO` is antiferromagnetic. On heating, the alignment is disturbed. The domains are randomized. `Ni^(2+)` has `d^(8)` configuration with unpaired electrons, hence, exhibits paramagnetism.