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

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Structure, bonding, and spectroscopyof actinides
in crystals A 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
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Structure, bonding, and spectroscopy of
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
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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

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f and d electrons in an octahedral field
Pa4 in Cs2ZrCl6
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f and d electrons in an octahedral field
U4 in Cs2ZrCl6
Pa4 in Cs2ZrCl6
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Structure, bonding, and spectroscopy of
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
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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

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Ab Initio Model Potentials as Effective
CoreEmbedding Potentials

• Non-parametric produced directly from the
  frozen orbitals
• Inactive-active explicit interactions
• Coulomb, Exchange, Linear independence

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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
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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
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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
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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
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Embedded Cluster Hamiltonian
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Spin-orbit coupling / electron correlation
Spin-orbit splittings
An aproximate decoupling of correlation and
spin-orbit
Use G space for the spin-free spectrum
Use P space for the spin-orbit couplings
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Spin-free state shifted Hamiltonian
large CI space G
small CI space P
Use G space for the spin-free spectrum
Use P space for the spin-orbit couplings
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Spin-free state shifted Hamiltonian
large CI space G
small CI space P
?
?
Use G space for the spin-free spectrum
Use P space for the spin-orbit couplings
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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 G space for the spin-free spectrum
Use P space for the spin-orbit couplings
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Details of the calculations

• Embedded-cluster (embedding AIMP for ionic
  solids)

• Effective core potential (Cowan-Griffin-Wood-Bor
  ing based AIMP)

• spin-free CASSCF/CASPT2

• spin-orbit sfss-SOCI MRCI(S)

• Embedding potentials

• Cluster (AnL6)q-

500 AIMPs 3000 point charges at
experimental sites so that E(R) is
stable
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Details of the calculations

• Embedded-cluster (embedding AIMP for ionic
  solids)

• Effective core potential (Cowan-Griffin-Wood-Bor
  ing 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
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Details of the calculations

• Embedded-cluster (embedding AIMP for ionic
  solids)

• Effective core potential (Cowan-Griffin-Wood-Bor
  ing based AIMP)

• spin-free CASSCF/CASPT2

• spin-orbit sfss-SOCI MRCI(S)

• SA-CASSCF

5f,6d,7sN

• MS-CASPT2

An 5d106s26p6 5f,6d,7sN 6 x Cl 3s23p6
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Details of the calculations

• Embedded-cluster (embedding AIMP for ionic
  solids)

• Effective core potential (Cowan-Griffin-Wood-Bor
  ing 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) CAS5f,6d,7sN
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Results type of results
Local structure (ground/excited states) bond
lengths, vibrational frequencies
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Results type of results
Local structure (ground/excited states) bond
lengths, vibrational frequencies
Wave functions (and their analyses)
bonding interactions
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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
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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
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Cs2NaYCl6Ce3 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
P25 kbar
P0
f1
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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
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Results accuracies (validation applications)
presumably (no EXAFS available)
Cs2NaYCl6
Bond distances
0.01Å
Cs2ZrCl6
very good (exceptions?)
Cs2ZrCl6Pa4 YAGCe3
Bond length changes
Cs2GeF6
Vibrational frequencies
Ce3,Pr3,Sm2,Pa4
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SrF2
Electronic transitions
Ce3,Pa4,U3,U4
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BaF2
Pressure induced shifts of electronic transitions
YAG (Y3Al5O12)
semiquantitative
Sm2
Intensidades relativas
semiquantitative
Ce3,U3,U4
CsCaBr3
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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
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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
YLiF4U4
YF3U4
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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

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Cs2GeF6U4, a potential quantum cutter or solid
state laser ?
1S0
5f2 levels
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Cs2GeF2U4, a potential quantum cutter or solid
state laser ?
quantum cutting orcascade luminescence
1S0
5f16d1 levels
5f16d1 levels
1S0
5f2 levels
3P0
5f2 levels
3H4
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Cs2GeF2U4, 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
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Absorption spectrum.
Miroslaw Karbowiak, University of Wroklaw
growth of Cs2GeF6U4 single crystals
experimental absorption spectrum (7 K)
broad, intense bands 37000 45000cm-1
most prominent at 38000 cm-1
no appreciable fine vibronic structure
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Absorption spectrum.
Theoretical spectrum
Five 5f16d1 origins 1A1g ? iT1u ( i 1,5)
2500cm-1 too high (0.3 eV) (7 )
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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
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Emission spectrum.
5f16d1 levels
1Eu
1T2g
5f2 levels
2T1g, 2T2g
3T2g
1T1g
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Emission spectrum.
Large Stokes shift 6200 cm-1
1Eu
1T2g
2T1g, 2T2g
3T2g
1T1g
1A1g
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Emission spectrum.
Spontaneous emission lifetime ?
Experiments underway
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An unexpected 5f17s1 manifold U-trapped
exciton?
Å
2.154, 2.174, 2.21
2.09
U(IV)
Bond length U(V) cluster
Very diffuse 7s orbital
Energy sensitive to basis set delocalization
U - trapped exciton ?
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An unexpected 5f17s1 manifold U-trapped
exciton?
Impurity-trapped exciton D. S. McClure, et
al. Phys. Rev. B, 32, 8465 (1985) SrF2Yb2
anomalous emissionThe 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
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Analysis of the wavefunctions
7s MO 5f17s1-23A2u (UF6Cs8)6
7s AO 5f17s1-3F U4
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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
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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 UO2Pu4, PuO2U4

• Transuranium systems (the f7 configuration)
• Cm3 in Cs2NaYCl6 (experimental
  spectroscopy available)
• and Am2 and Bk4

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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/
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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.

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