High temperature phase transitions and thermal decomposition of KH 2PO 4 crystals

Abstract

The Raman spectrum of KH 2PO 4 has been measured from room temperature to 250 °C to obtain better microscopic understanding of the high-temperature phase transitions and the thermal decomposition, where the existence and nature of both effects have yet to be firmly established. We found that after the onset of the thermal decomposition near 175 °C, two successive high-temperature phase transitions occur at around 195 and 215 °C. Clear evidence is given that information about the onset and progress of the thermal decomposition can be obtained from additional Raman lines corresponding to vibrations of (PO 3) n polyanions. The thermal decomposition could be much suppressed by using a glass seal or by controlling heating conditions and, even in these cases, high-temperature transitions are also observed to occur, revealing that it can occur independently irrespective of the partial dehydration. The high-temperature phases could be stabilized at room temperature in vacuum, but they revert slowly to tetragonal KH 2PO 4 in the open air within several days. Electrical conductivity measurements have also been carried out for several heating cycles with various heating rates.