The solid state isotopic exchange reaction between cationic and anionic Co atoms in [Co(H 2O) 6][Co EDTA] 2·4H 2O

Abstract

The solid state isotopic exchange reaction between cobalt atoms in cationic and anionic sites in the double complex [ 60Co(H 2O) 6][Co EDTA] 2·4H 2O was investigated in order to observe: (1) whether isotopic exchange reactions between atoms at the normal lattice sites of the double complex can occur as they do between dopant ions and atoms of the host matrices; (2) whether the thermodynamic equilibrium value of the fraction of activity exchanged is attained in this class of compounds, since attainment of the thermodynamic equilibrium conditions is rarely observed in doped crystals. It was found that in the above double complex the exchange between cobalt atoms in cationic and anionic sites has a negligible probability of taking place by cooperative motion in the ring of two cobalt atoms, one at a cationic site and the other in a neighbouring anionic site. The presence in the solid of vacant cobalt sites in anionic positions seems essential to the isotopic exchange. This conclusion was reached by comparing the isotopic exchange reaction in irradiated (100 Mrad) and unirradiated crystals. In neither case was the thermodynamic equilibrium value of the exchanged activity fraction attained, but the final fraction is higher by a factor 5·5 in irradiated than in unirradiated crystals. The isotopic exchange in unirradiated crystals occurs simultaneously with the dehydration of the compound which probably induces in the solid a defect concentration higher than that corresponding to the equilibrium value.