The influence of double (cationic/anionic) substitution upon He diffusion was studied in 2 apatite compositions. The He bulk diffusion constants were deduced from non-destructive 3He depth profiling, using the 3He(d,p)4He nuclear reaction. The results were obtained from sintered ceramics which were implanted with 3MeV 3He+ ions to a depth of around 9μm, and fluences of 1016/cm2, before annealing in air at 250 to 400C. It was shown that the activation energy for He diffusion, determined by using 2 different models, decreased with substitution. Thus, it was equal to 1.27eV for Ca10(PO4)6F2 and to 0.89eV for Ca4Nd6(SiO4)6F2. The activation energy for sintered Ca10(PO4)6F2 ceramics was similar to that reported for a single crystal having the same composition. This led to the conclusion that the diffusion mechanism involved tunnels in the structure whose size increased with substitution.

Effect of Composition on Helium Diffusion in Fluoroapatites Investigated with Nuclear Reaction Analysis. S.Miro, F.Studer, J.M.Costantini, J.Haussy, P.Trouslard, J.J.Grob: Journal of Nuclear Materials, 2006, 355[1-3], 1-9