HIGH-TEMPERATURE PHASE-TRANSITION IN POTASSIUM DIHYDROGEN PHOSPHATE CRYSTALS

Single crystals of potassium dihydrogen phosphate (KDP) were grown by slow evaporation from supersaturated aqueous solution. The crystal structure of KDP was identified by X-ray diffraction analysis. Simultaneous TG, DTG and DTA thermal analysis of KDP crystals over the temperature range 25-600° C showed that the potassium orthophosphate decomposed to potassium pyrophosphate and then to metaphosphate as end product. DTA studies at the higher heating rates of 50 and 100° C min-1 showed a clear resolution of the first peak into two peaks; the first was attributed to a phase transition at 200 °C and the second was due to the decomposition of the sample. It is interesting to note that the high-temperature phase transition for KDP was also obtained at 175° C by DSC studies at 2° C min-1. The activation energies for the thermal processes of KDP occurring at 230, 265 and 310°C were calculated as 155.56, 286.56 and 81.46 kJ mol-1 by the Kissinger method, and 164.01, 294.97 and 91.34 kJ mol-1 by the Ozawa method. X-ray diffraction and infrared absorption spectra confirmed the high-temperature phase transition obtained by DTA and DSC as well as the thermal decomposition of the KDP samples. © 1990.