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.