Actinide Behavior in Materials Stability and Electronic Properties

Actinide Behavior in Materials: Stability and Electronic Properties of Am-bearing Britholites Marjorie Bertolus, Mireille Defranceschi Commissariat à l’Energie Atomique Cadarache, Saclay, France 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 1

Context ü French law concerning nuclear waste management ü “New crystalline Host Phases” research program ü Synthetic ceramics analog of natural minerals e. g. apatite, zirconolite, hollandite ü Immobilization of separated long-life radioactive elements I, Cs, actinides: U, Pu, Np, Am, Cm ü Materials exposed to self-irradiation ü Experiments using simulants 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 2

Objectives of the Quantum Chemistry Study ü Fundamental study of potential host phases for radionuclides (RN) ü RN immobilization mechanisms ü Support to experiments ü Comparison of rare-earth elements and actinides ü Nature of bonding, especially between RN and anions ü Correlation to phase stability 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 3

Specificities of our Study ü Study of electronic properties • First-principles method ü Complex materials • Large unit-cells, little symmetry • Different types of bonds, including covalent ü Heavy element-bearing structures • Relativistic effects significant in REE and actinides • Few ab initio studies of Am-bearing solid systems • Problem of adequate pseudopotentials 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 4

Methods Used ü Density Functional Theory Generalized Gradient Approximation (GGA) ü Spin-polarization included ü Computation scheme validated on study of Nd, Ce and U ü Geometry optimization using plane waves and ultrasoft pseudopotentials. Code: CASTEP ü Cohesion energy calculations and charge density analysis using localized orbitals. Code: DMol 3 ü Validation of DFT on Am Vetere et al. : Chem. Phys. Letters 396, 452 (2004) 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 5

The Apatite Family: Structure of Fluorapatite Formula: Ca 10(PO 4)6 F 2 Hexagonal structure Ca Polyhedra d = 2. 57 Ǻ F Tunnels d = 2. 41 Ǻ 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 6

Am 3+ Substitution in the Apatite Structure Fluorapatite: Ca 10(PO 4)6 F 2 Ca 2+ + PO 43 - Am 3+ + Si. O 44 - Am-bearing Britholite Ca 9 Am(PO 4)5 Si. O 4 F 2 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 7

Energetic Calculations performed ü Am and Nd in cationic site 1 or 2 ü All configurations relative to Si. O 4 for Nd ü 3 configurations relative to Si. O 4 for Am 12 + 6 configurations Energetic ü Large stabilization of the Am-bearing britholites compared to fluorapatite ü Am-britholite more stable than Nd-britholite ü Substitution in site 2 energetically favorable for both RN 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 8

Geometry Analysis ü Unit-cell volume Am 3+ (2) < Fluorapatite < Am 3+ (1) Smallest volume for the most stable configuration ü RN-Si distances shortest for most stable configurations Local charge balance favorable ü Average distances between RN and first neighbors site 1 d (Am) < d (Nd) < d (Ca) Sites decrease in size site 2 d (Ca) < d (Am) < d (Nd) ü Ionic radii Am 3+ < Nd 3+ < Ca 2+ Explain why Am and Nd substitute in site 2 But not the volume variations between sites 1 and 2 Effect of the electronic structure? 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 9

Electronic Deformation Density Map in (OO 1) Plane Dr > 0 Dr < 0 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 10

Electronic Density : Mulliken Charges ü Charges on RN much lower than formal charges Am: 0. 66, Nd: 0. 95; compared to Ca: 1. 56 ü Charge decrease in % on RN neighbors compared to Ca O near Am 3+ O near Nd 3+ F near Am 3+ F near Nd 3+ 13 11 18 15 Charges on RN neighbors much lower than on Ca ones ü Significant charge transfer between RN and anions Stronger for Am than Nd Correlation with structure stability 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 11

Conclusion ü Am-bearing britholite very stable ü Am forms more covalent bonds than Nd ü Correlation between stability and covalence of bonds formed by radionuclide ü Nd and Am-bearing britholites exhibit similar properties Nd good simulant for Am 35èmes Jd. A, Baden bei Wien, April 23 -26, 2005 12