Structure, bonding, and spectroscopy of actinides in crystals A quantum chemical perspective

1. Structure, bonding, and spectroscopyof actinides in crystalsA quantum chemical perspective Zoila Barandiarán Departamento de Química & Instituto Universitario de Ciencia de Materiales Nicolás Cabrera Universidad Autónoma de Madrid, Spain. http://www.uam.es/zoila.barandiaran

2. Structure, bonding, and spectroscopyof actinides in crystalsA quantum chemical perspective Actinides advanced nuclear energy systems challenge basic and applied research societal interest: controversial energy source; security & waste problems in crystals ions in crystals, solid fuel and fission products (UO2, PuO2) extreme conditions (temperature, pressure) spectroscopy open shells: 5f, 6d, 7s large manifolds of excited states: 5fN, 5fN-1 6d1, and others spectroscopy: a basic tool expected/exotic electronic structures beyond the gs figerprints of local structure and bonding models of coordination chemistry quantum chemical perspective

3. U4+ in Cs2GeF6 Actinide ions doped in solids – an example point defect: + local distortion + new electronic states in the energy gap how many states ? how to calculate them ? N electrons formally in 5f, 6d shells in a crystal field

4. f and d electrons in an octahedral field Pa4+ in Cs2ZrCl6

5. f and d electrons in an octahedral field U4+ in Cs2ZrCl6 Pa4+ in Cs2ZrCl6

6. Structure, bonding, and spectroscopyof actinides in crystalsA quantum chemical perspective A quantum chemical model (for ground and excited states) Results an overview type of results accuracies a show case Conclusions and what is next

7. A quantum chemical model for ground and excited states • Relativistic (spin-orbit) • Electron correlation • Large fn and fn-1 d1 manifolds • relativistic core-AIMP (ECP) • wave-function based correlation methods • (CASSCF + MS-CASPT2) Defect cluster Embedding host embedding-AIMP fn , fn-1d1

8. Non-parametric & produced directly from the frozen orbitals Inactive-active explicit interactions Coulomb, Exchange, Linear independence Active (cluster valence) (UF6)2- 68 electrons Inactive (core) U [Kr],4f F 1s Material Cs2GeF6 with U4+ impurities Inactive (environment) Cs2GeF6 Ab Initio Model Potentials as Effective Core+Embedding Potentials

9. long-range local  local Embedded Cluster Hamiltonian Relativistic Cowan-Griffin-Wood-Boring Hartree-Fock all-electron atomic calculations + Frozen core approximation + AIMP recipe for representation of operators Coulomb Exchange + scalar relativistic Linear independence

10. short-range  spectral representation Embedded Cluster Hamiltonian Relativistic Cowan-Griffin-Wood-Boring Hartree-Fock all-electron atomic calculations + Frozen core approximation + AIMP recipe for representation of operators Coulomb Exchange + scalar relativistic Linear independence

11. Embedded Cluster Hamiltonian Relativistic Cowan-Griffin-Wood-Boring Hartree-Fock all-electron atomic calculations + Frozen core approximation + AIMP recipe for representation of operators Coulomb Exchange + scalar relativistic Linear independence

12. Embedded Cluster Hamiltonian Self-Consistent Embedded Ion calculations • Perfect crystal lattice • loop over lattice ions until convergence • perform a single embedded-ion calculation (SCF, CASSCF) • produce its embedding-AIMP out of its orbitals • update the lattice embedding potentials • end loop

13. Embedded Cluster Hamiltonian

14. depend on: spin-orbit couplings spin-free spectrum small CI space P large CI space G which demand: Spin-orbit coupling / electron correlation Spin-orbit splittings An aproximate decoupling ofcorrelationandspin-orbit Use Gspace for the spin-free spectrum Use Pspace for the spin-orbit couplings

15. Spin-free state shifted Hamiltonian large CI space G small CI space P Use Gspace for the spin-free spectrum Use Pspace for the spin-orbit couplings

16. Spin-free state shifted Hamiltonian large CI space G small CI space P   Use Gspace for the spin-free spectrum Use Pspace for the spin-orbit couplings

17. Spin-free state shifted Hamiltonian • Codes: MOLCAS Björn O. Roos et al., Lund University COLUMBUS Russ M. Pitzer et al., Ohio State University EPCISO Valérie Vallet et al., Université de Lille Use Gspace for the spin-free spectrum Use Pspace for the spin-orbit couplings

18. Details of the calculations • Embedded-cluster (embedding AIMP for ionic solids) • Effective core potential (Cowan-Griffin-Wood-Boring based AIMP) • spin-free: CASSCF/CASPT2 • spin-orbit: sfss-SOCI [MRCI(S)] • Embedding potentials: • Cluster: (AnL6)q- ~ 500 AIMPs + 3000 point charges atexperimental sites so that E(R) is stable

19. Details of the calculations • Embedded-cluster (embedding AIMP for ionic solids) • Effective core potential (Cowan-Griffin-Wood-Boring based AIMP) • spin-free: CASSCF/CASPT2 • spin-orbit: sfss-SOCI [MRCI(S)] • Core AIMPs: An: [Xe,4f] 5d,6s,6p, 5f,6d,7s Cl: [Ne] 3s,3p • Valence basis sets: An: (14s10p12d9f3g)/[6s4p5d4f1g] Cl: (7s7p1d)/[3s4p1d]

20. Details of the calculations • Embedded-cluster (embedding AIMP for ionic solids) • Effective core potential (Cowan-Griffin-Wood-Boring based AIMP) • spin-free: CASSCF/CASPT2 • spin-orbit: sfss-SOCI [MRCI(S)] • SA-CASSCF: [5f,6d,7s]N • MS-CASPT2: An: 5d106s26p6 [5f,6d,7s]N + 6 x Cl: 3s23p6

21. Details of the calculations • Embedded-cluster (embedding AIMP for ionic solids) • Effective core potential (Cowan-Griffin-Wood-Boring based AIMP) • spin-free: CASSCF/CASPT2 • spin-orbit: sfss-SOCI [MRCI(S)] • spin-free-state-shifted Spin-Orbit CI: Wood-Boring spin-orbit operator scaled by 0.9 Basis of double-group adapted functions MRCI(S) CAS[5f,6d,7s]N

22. Results: type of results Local structure (ground/excited states) bond lengths, vibrational frequencies

23. Results: type of results Local structure (ground/excited states) bond lengths, vibrational frequencies Wave functions (and their analyses) bonding interactions

24. Results: type of results Local structure (ground/excited states) bond lengths, vibrational frequencies Wave functions (and their analyses) bonding interactions Absorption/emission spectra transition energies, transition moments, emission lifetimes

25. Results: type of results Green-to-blue light upconversion in Cs2ZrCl6: U4+ Local structure (ground/excited states) bond lengths, vibrational frequencies Wave functions (and their analyses) bonding interactions UO22+ impurities U4+ impurities Absorption/emission spectra transition energies, transition moments, emission lifetimes Mechanisms of energy transfer upconversion/quantum cutting mechanisms 5f16d1 levels 5f2 levels

26. Cs2NaYCl6:Ce3+ under pressure Results: type of results Local structure (ground/excited states) bond lengths, vibrational frequencies Wave functions (and their analyses) bonding interactions Absorption/emission spectra transition energies, transition moments, emission lifetimes Mechanisms of energy transfer upconversion/quantum cutting mechanisms Pressure effects d(eg)1 d(t2g)1 P=25 kbar P=0 f1

27. Results: type of results Local structure (ground/excited states) bond lengths, vibrational frequencies Wave functions (and their analyses) bonding interactions Absorption/emission spectra transition energies, transition moments, emission lifetimes Mechanisms of energy transfer upconversion/quantum cutting mechanisms Pressure effects

28. Results: accuracies (validation + applications) presumably (no EXAFS available) Cs2NaYCl6 Bond distances 0.01Å Cs2ZrCl6 very good (exceptions?) Cs2ZrCl6:Pa4+ YAG:Ce3+ Bond length changes Cs2GeF6 Vibrational frequencies Ce3+,Pr3+,Sm2+,Pa4+ 5% SrF2 Electronic transitions Ce3+,Pa4+,U3+,U4+ 10% BaF2 Pressure induced shifts of electronic transitions semiquantitative Sm2+ YAG (Y3Al5O12) Intensidades relativas Ce3+,U3+,U4+ semiquantitative CsCaBr3

29. Results: a show case Predicting the luminescence of a new material + experimental & theoretical study U4+ in fluorides U4+ 5f2, 5f16d1 manifolds ~90 excited states fluorides large transparency window Potentiality as ● UV solid state laser ● Phosphor based on quantum cutting or cascade luminescence

30. UV solid state laser quantum cutting orcascade luminescence 5f16d1 levels 1S0 5f2 levels Weak, slow, two-step 5f→5f luminescence Strong, broad, fast 6d→5f luminescence YLiF4:U4+ YF3:U4+

31. U4+ in Cs2GeF6 UV solid state laser quantum cutting orcascade luminescence • The electronic structure of the 5f2 manifold • The 5f1 6d1 manifold • Promote the synthesis and experimental study • An unexpected 5f1 7s1 manifold: U-trapped excitons

32. Cs2GeF6:U4+, a potential quantum cutter or solid state laser ? 1S0 5f2 levels

33. Cs2GeF2:U4+, a potential quantum cutter or solid state laser ? quantum cutting orcascade luminescence 1S0 5f16d1 levels 5f16d1 levels 1S0 5f2 levels 3P0 5f2 levels 3H4

34. Cs2GeF2:U4+, a potential quantum cutter or solid state laser ? UV solid state laser 1S0 5f16d1 levels 5f16d1 levels 1S0 5f2 levels 5f2 levels Strong, broad, fast 6d→5f luminescence

35. Absorption spectrum. Miroslaw Karbowiak, University of Wroklaw • growth of Cs2GeF6:U4+ single crystals • experimental absorption spectrum (7 K) • broad, intense bands 37000 – 45000cm-1 • most prominent at 38000 cm-1 • no appreciable fine vibronic structure

36. Absorption spectrum. • Theoretical spectrum • Five 5f16d1 origins: 1A1g→ iT1u ( i = 1,5) • 2500cm-1 too high (0.3 eV) (7 %)

37. Absorption spectrum. • Theoretical spectrum • Five 5f16d1 origins: 1A1g→ iT1u ( i = 1,5) • 2500cm-1 too high (0.3 eV) (7 %) • Intensities: + most prominent band 1A1g→ 1T1u + relative intensities ok, - except for 1A1g→ 2T1u

38. Emission spectrum. 5f16d1 levels 1Eu 1T2g 5f2 levels 2T1g, 2T2g 3T2g 1T1g

39. Emission spectrum. Large Stokes shift: 6200 cm-1 1Eu 1T2g 2T1g, 2T2g 3T2g 1T1g 1A1g

40. Emission spectrum. Spontaneous emission lifetime:  Experiments underway

41. An unexpected 5f17s1 manifold: U-trapped exciton? Å 2.09 2.154, 2.174, 2.21 U(IV) • Bond length ~ U(V) cluster • Very diffuse 7s orbital • Energy sensitive to basis set delocalization U - trapped exciton ?

42. An unexpected 5f17s1 manifold: U-trapped exciton? Impurity-trapped exciton D. S. McClure, et al. Phys. Rev. B, 32, 8465 (1985)SrF2:Yb2+anomalous emission“The excited state ... could be called an impurity-trapped exciton, since it consists of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites...”“The trapped exciton geometry is probably that expected for a trivalent impurity ion, Yb3+...” Yb2+→ Yb3+ + 1e(Sr) very short bond length localised hole delocalised

43. Analysis of the wavefunctions 7s AO [5f17s1-3F U4+] 7s MO [5f17s1-23A2u (UF6Cs8)6+]

44. Microscopic description of an impurity trapped exciton • ~ U(V) bond length •Electronic density in the frontier of the UF6 unit • Hole localized in the U(5f) Diffuse orbitals of Ln/An in solids can lead to impurity trapped excitons

45. Conclusions Wavefunction based ab initio embedded cluster calculations on Lnq+ and Anq+ impurities in ionic hosts • Reliable enough (complement experiments, predict) • Can be used to progress in the understanding of Advanced Nuclear Energy Systems What is next ? Nuclear fuel and nuclear wastes materials • UO2 (experimental spectroscopy available) , PuO2 • diluted UO2/PuO2 mixtures UO2:Pu4+, PuO2:U4+ Transuranium systems (the f7 configuration) • Cm3+ in Cs2NaYCl6 (experimental spectroscopy available) • and Am2+ and Bk4+

46. Acknowledgments Noémi Barros Luis Seijo Belén Ordejón Ana Muñoz José Luis Pascual me José Gracia Fernando Ruipérez on campus, UAM 2006 Goar Sánchez in La Sierra, Madrid 2007 http://www.uam.es/quimica/aimp/

47. Acknowledgments • Miroslaw Karbowiak, Faculty of Chemistry, University of Wroclaw, Wroclaw, Poland • Norman Edelstein,Lawrence Berkeley National Laboratory, Berkeley, California, USA • Björn Roos, Rolandh Lindh,(MOLCAS)Lund University, Lund, Sweden • Russell Pitzer, (COLUMBUS) Ohio State University, Columbus, Ohio, USA • Valérie Vallet, Jean-Pierre Flament (EPCISO) Université de Lille, Lille, France • Spanish Ministry of Education and Science, DGI-BQU2002-01316,DGI-CTQ2005-08550.

48. Structure, bonding, and spectroscopy of actinides in crystals.A quantum chemical perspectiveUniversidad Autónoma de Madrid