Migration of cations in two solid-state reactions

Abstract

The quantitative use of Mössbauer spectroscopy in studying reaction kinetics is illustrated by the results obtained on the following two reactions: A, the rearrangement of kaolinite, to form mullite and cristobalite at 1,100°; B, the reaction between iron(III) oxide and zinc oxide to give zinc ferrite spinel (ZnFe₂O₄) at 800°. Both absorber and source methods have been used to identify the sites occupied by the cations at various stages during the reaction and to determine the rate at which the Mössbauer atoms move from one site to another. By comparing the Mössbauer results with X-ray spectroscopic data the following conclusions arise.

In the first reaction (i), impurity (⁵⁷Co) cations become associated with lattice sites at temperatures well below dehydration temperature; (ii) the cations are not firmly bound in material which can be well characterized as mullite by X-ray methods. This suggests that the cations take longer to attain their final positions than may be inferred from X-ray data. In both reactions A and B, the cations are still mobile after the oxygen lattice is established.