Raman and structural studies on the high-temperature regime of the KH$_2$PO$_4$-NH$_4$H$_2$PO$_4$ system

Abstract

We have studied the high-temperature phase transition (HTPT) of crystalline potassium and ammonium dehydrogen phosphates and solid solutions of them with composition (1-$x$)KH$_2$PO$_4+x$NH$_4$H$_2$PO$_4$ (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 T$_p$ 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 T$_p$). The presence of the polyphosphate vibrational bands assigned to the stretch vibration of its PO$_{2}$ species (at about 1120 \cm for pure KDP) accompanied by a broad band assigned to P-O-P backbone vibrations (at about 713 \cm for pure KDP) [14] become evident at temperature higher than T$_p$ depending on the composition of KADPx.