Kinetic process of dehydration at fusion temperature and the high-temperature phase transition in KH2PO4

X-ray diffraction and Raman spectroscopy analysis were used to study the kinetic process of dehydration and the high-temperature phase transition (HTPT) in KH2PO4. The results show strong evidence that KH2PO4 presented two phase transitions at about Ttr = 453 K and 483 K. The new Raman bands, which were observed in the KDP (phase (Ia) and phase (Ib)) spectra (at 128, 152, 372, 437, 540, 904, 926, 966, 1100, 1156 and 1171 cm−1), did not correlate to any of the highly intense peaks present in KPO3 (383, 671, and 1139 cm−1), K2H2P2O7 (705 and 1041 cm−1), and the KnH2PnO3n+l (n ≫ l) polymer (294 cm−1) spectra, indicating that a number of structural changes took place. Accordingly, the results presented in the current work corroborate the findings of former studies attributing the phenomena at high temperature in KDP to structural phase transition rather than thermal decomposition.

► We have studied the increase of temperature from 303 to 503 K in the Raman spectra of fresh KDP. Two phase transitions were noted at 453 K and 483 K.
► We have studied the Raman spectroscopy and X-ray diffraction pattern of KDP heated till softening and the dehydrated salts prepared at the fusion of KDP, for 1, 2, 3, 4, and 5 min.
► We have observed that the dehydration of KDP was progressive and that the product was a blend of KPO3, K2H2P2O7, and KnH2PnO3n+l (n >>l) polymer.