Orientational disorder: A mechanism of amorphization at high pressure

Amorphization generally arises due to chemical disorder, high defect densit y or kinetic hindrances of equilibrium phase transitions. In this paper, we report that it can also be driven by orientational disorder of polyatomic ions or units. In potash alum, some of the sulphate ions are misoriented wi th their S-O bonds pointing towards potassium rather than towards aluminum, leading to an "orientational disorder" which ranges from 10 to 24%. Raman spectroscopic and X-ray diffraction studies reveal that samples with high i nitial orientational disorder exhibit a "reversible" amorphization at press ure around 60 kbar whereas those with low initial disorder exhibit an "irre versible" structural transition to a disorder-free crystalline phase.