Phase transitions in ammonium nitrate

The phases of ammonium nitrate, NH₄NO₃, doped with small amounts of NiO or CuO have been studied by X-ray and neutron diffraction at various temperatures from 80 to 423 K. NH₄NO₃ with metal-oxide additives, which are believed to form complexes, exists as a solid solution. Phase IV is stable down to 140 K in the NiO-doped samples and 210 K in the CuO-doped sample. The thermal expansion of this phase is extremely anisotropic, with the b axis expanding rapidly with increasing temperature while the a axis contracts slightly. At higher temperatures the doped samples transform directly from phase IV to phase II, bypassing phase III, starting at about 328 K. The structure of phase II, which is disordered, was refined in the space group P4/mbm. The structure of phase III contains hydrogen-bonded chains parallel to the b axis.