In situ synchrotron diffraction studies on the formation kinetics of jarosites

Abstract

This paper reports the results of time-resolved synchrotron powder diffraction experiments where jarosites with different K/H3O, K/Na and Na/H3O ratios were synthesized in situ at temperatures of 353, 368 and 393 K in order to observe the effect on kinetics and species produced. The Na/H3O sample formed monoclinic jarosite at all three temperatures, whereas the K/H3O and K/Na samples formed as rhombohedral jarosites at 353 K, and as mixtures of rhombohedral and monoclinic jarosites at the higher temperatures. The relative amount of the monoclinic phase increased with increase in temperature. Unit-cell parameter changes with reaction time could be explained by changes in iron stoichiometry (samples become more stoichiometric with time) together with changes in K/H3O and Na/H3O ratios. The reaction kinetics have been fitted using a two-stage Avrami model, with two different Avrami exponents corresponding to initial two-dimensional growth followed by one-dimensional diffusion-controlled growth. Activation energies for the initial growth stage were calculated to be in the range 90-126 kJ mol-1. © 2013 International Union of Crystallography.