Isotopic Exchange Reaction Kinetics in Mixed-Valence TI₄CI₆

Abstract

Kinetics new to the field of isotopic exchange in solids and based on diffusion controlled reactions are applied to isochronal and isothermal data obtained in a radiochemical study of two batches of ,04 Tl-labelled mixed-valence T1 4 C1 6 , consisting of either untreated or of crushed crystals. The exchange between Tl 1 and Tl m is shown to be faster in the latter samples. On the basis of previous results, suggesting that exchange would be triggered by the diffusion of anion vacancies, two models are probed, implying either the presence of two regions in the lattices (surfaces and dislocations as opposed to the bulk) or of two populations of vacancies (extrinsic and intrinsic). The mathematical expressions used allow a very good fitting of the data, much better than what is obtained with the classical MACKAY equations. However, in either case all derived parameters have quite acceptable values on physical grounds, except the reaction radius which appears to be too large, at a few nm. Ignoring this anomaly, the main difference between untreated and crushed crystals appears to be a lower apparent activation energy for diffusion in the latter, ascribable to the presence of strains in the lattice; a minor difference would arise from either a larger proportion of the disordered regions or a larger extrinsic vacancy concentration in the crushed samples, although both properties would contribute only to a few percents in the overall exchange process.

Synthesis and characterization of antimony mixed-valence compounds in view of solid state isotopic exchange studies

“…2,6 The most extensive work 7 11 relates to TII/T1111 exchange in TI(I)3TIIIIc16 which was examined by using both radiochemical and physical methods, and where a diffusion-controlled kinetics triggered by the presence of anion vacancies have been evidenced. 12 In this context, the mixed-valence compounds of antimony appear quite promising. Only one study has been reported on Cs4(SblIIc16)(SbVC16) and CSlo(SblIIc16)3(SbVC16).…”