Theoretical Study of Pu and Cs Incorporation in a Mono-silicate Neodymium Fluoroapatite Ca9Nd(SiO4)(PO4)5F2

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2000-06, Vol.104 (22), p.5380-5387
• Main Authors: Meis, C, Gale, J. D, Boyer, L, Carpena, J, Gosset, D
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp000096j

In the context of nuclear waste disposal, the possibility of incorporating selectively plutonium and cesium in a mono-silicate neodymium-fluoroapatite Ca9Nd(SiO4)(PO4)5F2 (britholite) is investigated. For this purpose, a force field for the modeling of fluoroapatite Ca10(PO4)6F2 is established based upon the experimental data of crystallographic parameters and elastic constants. To test further the validity of the potentials, the constant pressure specific heat for Ca10(PO4)6F2 is measured and compared to the calculated values. Neodymium ions and silicate groups are introduced in the fluoroapatite structure using transferable potentials to obtain the neodymium-britholites Ca10 - y Nd y (SiO4) y (PO4)6 - y F2. The force field accuracy is tested by comparing the calculated and experimental values for the lattice parameters and volumes with respect to y (1 ≤ y ≤ 6). Plutonium and cesium are introduced separately in Ca9Nd(SiO4)(PO4)5F2 also using transferable potentials. The bulk modulus B and the specific heats, C P and C V , of the resulting structures are calculated. The cases of trivalent and tetravalent plutonium have been studied by considering, respectively, the following compositions: Ca9Nd1 - x (Pu3+) x (SiO4)(PO4)5F2 and Ca9.25(Pu4+)0.25Nd0.5(SiO4)(PO4)5F2. In both cases plutonium is found to substitute for calcium Ca(2), which is seven coordinated, by six oxygen ions and a fluorine. Conversely, cesium, which is introduced in the following structure Ca9.5Cs0.5Nd0.5(SiO4)(PO4)5F2, will substitute for Ca(1) as well as for Ca(2), with a slight preference for Ca(1) which is nine oxygen coordinated. The activation energies for the lattice migration of Pu and Cs are also calculated in the presence of calcium vacancies.