Thermal decomposition of KIO4 and NaIO4 in relation to solid-state isotopic exchange reactions

Abstract

The thermal decomposition of KIO₄ and NaIO₄ has been studied. The decomposition product, KIO₃ or NaIO₃, suffers a very efficient exodiffusion during heating in such a way that the partly decomposed crystals consist of two parts: the outer part is a fragile layer of almost pure iodate, while the inner part is a solid crystalline lattice of almost pure periodate. The dimensions of the latter are not affected by the decomposition, so that its density decreases in proportion to the decomposed fraction. The decomposition isotherms present a saturation at long heating times which is not due to atmospheric effects, but is suppressed when the heating is performed with continuous removal of the outer layer. Kinetic analysis of the data shows a remarkable continuity between the exchange process, occurring at lower temperatures, and the decomposition. In particular, the dimensionality of the reaction, n, from the Erofeev equation, ranges in KIO₄ roughly from 1 to 2 with increasing temperature in the former case, then from 2 to 3 in the latter. These values are systematically higher by one unit in NaIO₄, indicating that the nucleation process, which creates the driving defect, cannot be neglected as it can for KIO₄. Both exchange and decomposition are dominated by the exodiffusion of some combined vacancy type defect, promoting the exodiffusion of the iodate and eventually triggering the decomposition; at sufficiently high temperature, the latter process becomes autocatalytic and develops three-dimensionally (n= 3 or 4).