New phase of ammonium nitrate: A monoclinic distortion of AN-IV.

A new phase of ammonium nitrate (AN) is found using first principles evolutionary crystal structure search. It is this polymorph that is associated with the phase transition to previously unidentified phase, which was detected in experiment at 17 GPa upon appearance of the two extra peaks in Raman spectrum. The new phase has a monoclinic unit cell in the P21/m space group symmetry (AN-P21/m) and is similar to the known phase IV of AN (AN-IV) except the ammonium molecules are oriented differently relative to the nitrate molecules. The calculated free energy of AN-P21/m is found to be lower than AN-IV at pressures above 10.83 GPa. The equation of state of both AN-P21/m and AN-IV phases (volume vs hydrostatic pressure at room temperature) has been obtained within the quasi-harmonic approximation. The calculated Raman spectrum of both AN-P21/m and AN-IV as a function of pressure is in a good agreement with experiment. The energetic competitiveness of AN-IV and AN-P21/m at ambient conditions suggests a possibility of the phase transition in a small pressure-temperature range near ambient pressure and temperature.