Computational study of Cs immobilization in the apatites Ca10(PO4)6F2, Ca4La6(SiO4)6F2 and Ca2La8(SiO4)6O2 -: art. no. 085110

Incorporation and lattice migration of cesium in three apatites fluorapatite Ca-10(PO4)(6)F-2, lanthanum-fluorobritholite Ca4La6(SiO4)(6)F-2 and lanthanum-oxyapatite Ca2La8(SiO4)(6)O-2 have been studied theoretically. The above apatites have been first optimized by applying the ab initio Hartree-Fock method using electron core pseudopotentials. Mulliken analysis has shown the calculated anion charges in the apatite tunnels to have formal values, thus establishing the presence of ionic bonds with cations at site II (6h), particularly in Ca2La8(SiO4)(6)O-2. These results have been used for optimizing the corresponding interactions in the interatomic potential method employed in the following. Free energy calculations have shown the preferential site for Cs and La incorporation in the three apatites to be site I (4f) and site II (6h), respectively. The calculated activation energies for Cs migration offer evidence that cesium diffusion is mainly controlled by intersite (I --> II) jumps to adjacent vacancies. Ca2La8(SiO4)(6)O-2 proves to have the higher immobilization capacity, because of the high activation energies characterizing all the possible lattice diffusion mechanisms.