[13Chad]

I haven't been able to find a very good explanation for this so I hope that someone can help to explain why NH₄F has the wurtzite structure, unlike other ammonium halides which possess CsCl or NaCl structure depending on the temp.
-besides hydrogen bonding...?

Also, if someone can also explain why PH₄I is the most stable of the PH₄X halides with respect to decompostion to PH₃ and HX

-mol.biologist trying to learn some inorganic chem, which is darn intimidating...so thank you in advance

[shelanachium]

Ammonium fluoride is strongly hydrogen-bonded, since two highly electronegative small atoms, N and F, are involved. It contains N -H....F hydrogen bonds. As the NH4+ ion is tetrahedral, there are four such bonds directed to the corners of a regular tetrahedron. A similar situation is found in ice, which also has a wurtzite-like structure. In the higher ammonium halides, the larger halide ions have too diffuse a negative charge to cause strong hydrogen bonding, and the NH4+ ions are freely rotating and behave as spheres like alkali cations. Hence structures are like those of alkali halides.

PH4I is the stablest phosphonium halide because HI is the strongest acid. This is both because of the large size of the I- ion (hence weaker attraction to proton) and the weakness of the H-I bond compared to HF, HCl and HBr.

[AWK]

I haven't been able to find a very good explanation for this so I hope that someone can help to explain why NH₄F has the wurtzite structure, unlike other ammonium halides which possess CsCl or NaCl structure depending on the temp.
-besides hydrogen bonding...?

Also, if someone can also explain why PH₄I is the most stable of the PH₄X halides with respect to decompostion to PH₃ and HX

-mol.biologist trying to learn some inorganic chem, which is darn intimidating...so thank you in advance

Structure of ionic compounds strongly depends on relative size of ions, and, what was pointed out by shelanachium on interaction between ions in the crystal. Additional factor which should be taken into account is a symmetry of ammonium cation which force a specific directional interactions.