Pressure-induced amorphization and orientational disorder in potash alum

Pressure-induced amorphization in potassium aluminium sulphate dodecahydrate (potash alum) has been studied using Raman spectroscopy in a diamond anvil cell up to a pressure of 15 GPa. In potash alum, some of the sulphate ions are misoriented with their S-O bond pointing towards potassium rather than aluminium, leading to an 'orientational disorder' which ranges from 10 to 24% at ambient conditions. The disorder is quantified from the intensities of the Raman lines characteristic of the two orientations. The samples with low initial disorder exhibit a sequence of two structural phase transitions occurring at 1.5 and 9 GPa respectively. The phase above 1.5 GPa, which could be pressure quenched to ambient conditions, is found to be free from orientational disorder. On the other hand, in the samples with high initial disorder, the disorder is found to grow as a function of pressure and beyond a critical value the system turns amorphous, which is confirmed from the disappearance of sharp diffraction peaks. In view of these results it is apparent that the orientational disorder is the driving mechanism of amorphization in potash alum. The different initial disorders in different samples are believed to arise from a combination of a dynamic disorder (equilibrium) and a static disorder arising from the defects. This defect could possibly be a missing molecule in the water octahedra around the potassium ion.