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 KAI(SO4)(2)center dot 12H(2)O to amorphous phases began at around 75 degrees 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 KAI(SO4)(2) appeared at approximately 180 degrees 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 KAI(SO4)(2)center dot 12H(2)O. (C) 2014 Elsevier B.V. All rights reserved.