Thermal dehydration of potash alum studied by Raman spectroscopy and X-ray diffraction analysis

•The thermal dehydration of potash alum proceeds through several steps.•Raman spectra and X-ray diffraction reveal the amorphization of the heated samples.•A transition from the amorphous phase to the KAl(SO4)2 crystal phase is observed in the sample heated at 180 °C.

The thermal dehydrations of potash alum caused by heating at various temperatures for at least 2 h were investigated by ex situ Raman spectroscopy and X-ray diffraction (XRD) analyses in air. With increasing the heating temperature, all Raman peaks were observed to broaden, while an additional broad peak appeared at approximately 1030 cm−1 and shifted toward higher wavenumbers. In addition, the Raman band assigned to the O–H stretching mode weakened. The orientational disorder (OD) of the sulfate ions, as indicated by the intensity ratio of doublet peaks at 989 and 974 cm−1, was found to increase with increasing the heating temperature. The XRD patterns demonstrated that a structural phase transition from crystalline KAl(SO4)2⋅12H2O to amorphous phases began at around 75 °C, while broadening of the Raman peaks and an increase in OD also suggested the onset of an amorphous phase. Raman peaks corresponding to anhydrous KAl(SO4)2 appeared at approximately 180 °C. It was concluded that the elimination of water molecules was responsible for increase in the extent of OD, and this in turn induced the observed phase transitions. The formation of the amorphous phases observed in this work was similar to the pressure-induced amorphization of KAl(SO4)2⋅12H2O.