Why ammonium nitride is not used in preparation of NH4?

Answer 1

You should be aware the ammonium is NH4. If you already have ammonium nitride, you don't need to prepare NH4 because you already have it, in a water soluble compound. Admittedly, ammonium nitrate is used more commonly than the nitride.

Answer 2

Ammonium nitride does not exist in practical terms; it is thermodynamically unstable and prone to decomposition. It reacts readily with water to form ammonia and nitrogen gas. Therefore, it is not a viable precursor for the preparation of ammonium ion (NH4+).