Optimizing the formula of rare earth‐bearing materials: A computational chemistry investigation

We present a computational investigation into the nature of bonds formed by rare earth elements (REE) in materials. This study focuses on the incorporation of neodymium in minerals called apatites, which are derived from fluorapatite: Ca₁₀(PO₄)₆F₂. These minerals, which allow many substitutions on all three Ca, P, and F sites, are considered as potential host phases for radioactive elements separated from nuclear waste. Nd and trivalent actinides have very similar physical and chemical properties, and Nd is not radioactive and much more easily handled. It is therefore very often used as a surrogate for actinides with oxidation degree three in experimental studies. Several formulas can be considered to substitute Nd³⁺ to Ca²⁺ and maintain charge balance of the apatite. Existing experimental and theoretical studies, however, mostly concern the Ca₉Nd(PO₄)₅SiO₄F₂ formula, where the Nd incorporation is compensated by the replacement of one PO by a SiO group. Moreover, only the cation position has been studied, whereas the silicate position and its influence on stability are unknown. We present a more general investigation of possible charge compensations on the one hand, and of the various resulting configurations on the other. All possible configurations of the two formulas Ca₉Nd(PO₄)₅ SiO₄F₂ and Ca₈NdNa(PO₄)₆F₂ have been considered. Calculations have been performed within the framework of density functional theory (DFT). A computation scheme that permits good accuracy in these systems within reasonable computation times is determined. The results obtained for cohesion energies, geometries, and electronic densities are discussed. As for the formulation, it is shown that the Ca₈NdNa(PO₄)₆F₂ formula is less stable than the fluorapatite, while Ca₉Nd(PO₄)₅ SiO₄F₂ is more stable. For the structures, it is found that Nd substitutes preferably in the second cationic site. Moreover, the most stable structures exhibit the shortest Na–Nd or Nd–Si distances. Local charge balance therefore seems favorable. Then, it is shown that Nd forms covalent bonds both in apatite and in britholite, while Na forms ionic bonds. Finally, a first correlation between the material stability and the covalent character of the bonds formed is established. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007