J. Phys. Colloques
Volume 37, Numéro C7, Décembre 1976Second International Conference on Lattice Defects in Ionic Crystals / Seconde Conférence Internationale sur les Défauts de Réseau dans les Cristaux Ioniques
|Page(s)||C7-540 - C7-540|
J. Phys. Colloques 37 (1976) C7-540-C7-540
CHEMICAL EFFECTS OF THE 79Br(?, 2n) 77Br NUCLEAR REACTION IN K2ReBr6-K2ReCl6 MIXED CRYSTALSH. MÜLLER and S. MARTIN
Chemical Laboratory, University Freiburg D-7800 Freiburg, Albertstr. 21, FRG
In continuation of investigations of the chemical fate of (n, ?) and (?, n) produced Re recoil atoms in K2ReBr6-K2SnCl6 [1, 2] and K2ReBr6-K2OsCl6  mixed crystals and IT and (n, ?) produced Br recoil atoms in K2ReBr6-K2SnCl6  and K2ReBr6-K2ReCl6  mixed crystals we have investigated recently the chemical effects of 77Br recoil atoms in the K2ReBr6- K2ReCl6 system activated by the nuclear reaction 79Br (?, 2n)77Br. By using ion exchange column chromatography as a new method of separation of the mixed Re77BrBr5-nCl2-n species larger amounts of the irradiated material could be separated following the dissolution. The yield of Re77BrBr2-5 increases, the yield of Re77BrCl2-5 decreases significantly with increasing K2ReBr6 concentration, all the other mixed hexabromochlororhenates and the free bromide-(77Br) are only little dependent of the mixed crystal composition at least at concentrations below 50 mol-% K2ReBr6. For zero K2ReBr6 content we obtained by extrapolation : 77Br- 16 % Re77BrBr2-5 3 % Re77BrBr4Cl2- 5 % Re77BrBr3Cl2-2 5 % Re77BrBr2Cl2-3 6 % Re77BrBrCl2-4 16 % Re77BrCl2-5 49 % The total bromine content (active 77Br and inactive 79,81Br) of the recoil labelled species amounts to 31 % at zero K2ReBr6 concentration clearly indicating that on the average two bromide ligands (one of them being 77Br, the other inactive) of the original six survive the nuclear process and are still bonded to their original Re central atom. Especially the appearance of the more intimately mixed species cannot be explained by models discussed hitherto in the literature  (billiard ball impact, ligand vacancy exchange followed by combination , hot spot reaction). We propose here a new mechanism which we want to call the Inverse Cage Effect. Following this model Br recoil atoms that are directed towards the central atom transfer their kinetic energy via the central atom to all or part of the other ligands which are driven in the surrounding lattice. The newly formed vacancies are then filled by halide ions from the environment including the ligands just pushed away. The experimental results may be summarized as follows : Besides a very small yield of primary retention (Re77BrBr2-5) one third of the recoil bromine atoms react following the inverse cage effect, two third are caught as interstitials (77Br-) or undergo simple displacements (Re77BrCl2-5). The recoil energy of the 77Br is estimated to be not larger than ca. 1 keV. This suggests that the nuclear reaction 79Br(γ, 2n)77Br is far from a compound nuclear mechanism.