Why is (tri-)ammonium phosphate ((NH4)3PO4) so unstable?

Why is triammonium phosphate, $\ce{(NH4)3PO4}$, so much more unstable than diammonium phosphate, $\ce{(NH4)2HPO4}$, or monoammonium phosphate, $\ce{NH4H2PO4})$?





> 
> Ammonium phosphates are solids between 0 and 75°C. **Because ammonium
>  hydroxide $(\ce{NH4OH})$ is a much weaker base than the common metal
>  hydroxides, ammonium phosphates are comparatively unstable.** Both
>  triammonium phosphate ($\ce{(NH4)3PO4}$) and the double salt
>  ($\ce{(NH4)3PO4.2(NH4)2HPO4}$) are unstable at room temperature and
>  evolve ammonia to form diammonium phosphate. Even the commercial mono-
>  and diammonium phosphates exhibit an ammonium vapor pressure in the
>  solid form and in solution. Although MAP is more stable than DAP, it
>  decomposes at high temperature to give ammonia and polyphosphoric
>  acid. Diammonium phosphate decomposes to give ammonia and monoammonium
>  phosphate at around 70°C.
> 
> 
> 


**Source:** (2005) Ammonium Phosphates and Ammonium Sulfate. In: Synthetic Nitrogen Products. Springer, Boston, MA. [DOI: 10.1007/0-306-48639-3\_12](https://doi.org/10.1007/0-306-48639-3_12).


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