Raman and structural studies on the high-temperature regime of the KH2PO4-NH4H2PO4 system

We have studied the high-temperature phase transition (HTPT) of crystalline potassium and ammonium dehydrogen phosphates and solid solutions of them with composition (1-x)KH2PO4 + xNH4H2PO4 (KADPx, for x =0.0, 0.1, 0.2, 0.3, 0.4, 0.6 and 1.0), by means of X-ray diffraction analysis at room temperature and in situ Raman spectroscopy as function of temperature. Analysis of the Raman spectra made it possible to monitor the temperature dependence of vibrational bands associated to structural changes taking place during a proposed partial dehydration reaction that starts to take place at a characteristic temperature Tp and tends to increase above it. This assignment is supported using characteristic vibrational bands of phosphates and polyphosphates (produced as a consequence of the partial dehydration reaction of the crystals above Tp). The presence of the polyphosphate vibrational bands assigned to the stretch vibration of its PO2 species (at about 1120 cm-1for pure KDP) accompanied by a broad band assigned to P-O-P backbone vibrations (at about 713 cm-1for pure KDP) [14] become evident at temperature higher than Tp depending on the composition of KADPx.