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.