Structural properties of ammonium perchlorate compressed to 5.6 GPa

The effect of pressure to 5.6 GPa on the structure of ammonium perchlorate (AP, NH4ClO4) was investigated at room temperature using X-ray diffraction studies and IR and Raman spectroscopy. Two phase transitions were observed. Under hydrostatic conditions, the first transition starts at about 0.9 GPa with the appearance of a few new diffraction peaks and a discontinuity in the pressure-induced shifts of certain Raman frequencies. This transition is most likely a molecular change in AP due to freezing of freely rotating NILI units, similar to effects at low temperature. The second transition starts at 2.9 GPa with a drop in diffraction intensity followed by a new pattern of diffraction peaks by 3.0 GPa. This transition is also characterized by another discontinuity in the Raman shifts and the emergence of two new Raman modes. The phase above 3.0 GPa is stable to 5.6 GPa. The P-V data to 0.9 GPa obtained from X-ray diffraction measurements were used to calculate a bulk modulus of 15.2 +/- 0.3 GPa for the ambient-pressure phase.