Raman study of orientational dynamics of sulphate ions in potash alum Dattagupta

Abstract

The A_g internal mode of the sulphate ion in potash alum is studied by Raman scattering as a function of temperature. This mode shows a splitting on account of the fact that a small fraction of S-O bonds point towards K⁺ while a larger fraction point towards Al³⁺. The area under each peak is a measure of the number of sulphate units having the corresponding orientation. Above 200K, the observed area ratio follows the expected exponential dependence on the temperature, and may be understood on the basis of phonon-assisted reorientations of the sulphate ion between the two configurations. At lower temperatures, there is a marked deviation from the exponential form. This is explained by invoking a temperature-independent contribution to the orientation rate. A detailed lineshape analysis is made and compared with experiment. The possible origin of the temperature-independent relaxation mechanism is also discussed.