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Topic: Wurtzite structure of ammonium fluoride  (Read 7305 times)

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Offline 13Chad

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Wurtzite structure of ammonium fluoride
« on: October 30, 2007, 12:43:20 PM »
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

Offline shelanachium

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Re: Wurtzite structure of ammonium fluoride
« Reply #1 on: November 07, 2007, 02:24:51 PM »
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.

Offline AWK

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Re: Wurtzite structure of ammonium fluoride
« Reply #2 on: November 08, 2007, 01:43:06 AM »
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.
AWK

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