Ammonium Nitrate - Crystalline Phases

Crystalline Phases

Transformations of the crystal states due to changing conditions (temperature, pressure) affect the physical properties of ammonium nitrate. The following crystalline states have been identified:

System Temperature (°C) State Volume Change (%)
- > 169.6 liquid -
I 169.6 to 125.2 cubic +2.1
II 125.2 to 84.2 tetragonal −1.3
III 84.2 to 32.3 α-rhombic +3.6
IV 32.3 to −16.8 β-rhombic −2.9
V −16.8 tetragonal -

The type V crystal is a quasi-cubic form which is related to caesium chloride, the nitrogens of the nitrates and the ammoniums are at the sites in a cubic array where Cs and Cl would be in the CsCl lattice. See C.S. Choi and H.J. Prask, Acta Crystallographica B, 1983, 39, 414-420.

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