The incorporation and lattice migration of Cs in the apatites was studied theoretically. The apatites were first optimized by using the ab initio Hartree-Fock method, with electron core pseudopotentials. A Mulliken analysis showed that the calculated anion charges in the apatite tunnels had values which indicated the presence of ionic bonds with cations at site II(6h); especially in La8Ca2(SiO4)6O2. These results were used to optimize the corresponding interactions in the interatomic potential method. Free-energy calculations showed that the preferential site for Cs and La incorporation in these apatites were site I(4f) and site II(6h), respectively. The calculated activation energies for Cs migration suggested that Cs diffusion was controlled mainly by intersite (I ↔ II) jumps to adjacent vacancies. It was found that La8Ca2(SiO4)6O2 had the highest immobilization capacity, because of the high activation energies which characterized all of the possible lattice diffusion mechanisms.

Computational Study of Cs Immobilization in the Apatites Ca10(PO4)6F2, Ca4La6(SiO4)6F2 and Ca2La8(SiO4)6O2. A.Chartier, C.Meis, J.D.Gale: Physical Review B, 2001, 64[8], 085110 (9pp)