High Temperature Phase Transitions of NH4H2PO4

The high temperature phase behaviour of NH4H2PO4 (ADP) was studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermogravimetric (TG) analysis in the temperature range of 300 to 500 K. The DSC results show peaks at 430 and 483 K, that are associated with different phase transformations. The powdered XRD results show that when an ADP sample is heated above 430 K, additional peaks appear besides those corresponding to its tetragonal structure at room temperature. As a consequence, the observed change of symmetry of the crystal suggests a phase transition like phenomena named as high-temperature phase transition (HTPT) at the characteristic temperature Tp = 430 K. However, the observed weight loss in the sample found in the TG curve, suggests that Tp marks the onset of partial polymerization taking place on the surface of the sample, such that the phase above 430 K is a mixed phase consisting of tetragonal ADP in the bulk of the sample and an ammonium polyphosphate (i.e., (NH4)2H2P2O7) at the surface of the crystal.