Study of the 5f electronic states in u, np, pu and am and (u,pu) oxides using high resolution xanes

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Study of the 5f electronic states in u, np, pu and am and (u,pu) oxides using high resolution xanes

P. Martin, D. Prieur, R. Bes, R. Belin, M. Strach, D. Manara, C. Valot, T. Vitova, T. Prussman, K. Dardenne, et al.

To cite this version:

P. Martin, D. Prieur, R. Bes, R. Belin, M. Strach, et al.. Study of the 5f electronic states in u, np, pu and am and (u,pu) oxides using high resolution xanes. NuMat2016 The Nuclear Materials Conference, Nov 2016, Montpellier, France. �cea-02437087�

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Context and Objectives

Methods

Study of the 5f electronic states in U, Np, Pu and Am and (U,Pu) oxides using

high resolution XANES

Ph. M. Martin

1

, D. Prieur

2

, R. Bès

3

, R. Belin

4

, M. Strach

1

, D. Manara

2

, Ch. Valot

1

, T. Vitova

5

, T. Prüßmann

5

, K. Dardenne

5

, J. Rothe

5

1-CEA, DEN/DTEC/SECA/LCC, Bagnol sur Cèze, France; 2-European Commission, JRC-Karlsruhe, Germany; 3-Antimatter and Nuclear engineering, Department of applied Physics, Aalto University, Finland; 4- CEA, DEN/DEC/SESC/LLCC, Cadarache, France; 5-Karlsruhe Institute of Technology, Institute for Nuclear Waste

Disposal, Germany

Uranium−plutonium mixed oxide fuels U_1−yPu_yO_2−x are currently studied within the frame of the fourth generation (GEN-IV) of nuclear reactors and more specifically for sodium-cooled fast neutron reactors (SFRs). Because of their specific neutronic spectrum, SFRs will also be able to burn long-lived minor actinides (MAs) such as Am and Np. One of the considered options is to add them homogeneously to the fuel in significant amounts (2−6%). In such complex mixed oxide systems, both the homogeneity of the cation distribution into the fluorite UO₂ structure and the oxygen stoichiometry significantly affects most of the fuel properties (thermal conductivity, melting temperature, diffusion phenomena, etc.). These properties depend mainly on the complexity of actinide electronic structures, in particular the unfilled actinide 5f valence shells. The latter can be directly probe with High Resolution X-ray spectroscopy performed at the M_4,5 (3d→5f transition) edges of actinides. Here, we show results obtained on binary oxides (UO₂, NpO₂, PuO₂ and AmO₂) and U_1-yPu_yO₂ (y=0.24, 0.44 and 0.62) samples combined with theoretical calculations with FDMNES [1] code.

[1] Bunau, O.; Joly, Y. J. Phys.: Condens. Matter 2009, 21, 345501 ; [2] T. Vitova et al., J. Phys. Conf. Ser. 430 (2013) 012117 ; [3] J. Rothe et al., Rev. Sci. Intrum. (2012) 043105 ; [4] T. Truphémus et al., J. Nucl. Mater. 432 (2013) 378–387 ; [5] Y. Lu et al., J. Nucl. Mater. 441 (2013) 411 ; [6] R. Vauchy et al. , Inorg. Chem. 55 (2016) 2123.

To overcome the core-hole limitation in standard XANES measurements , an emission spectrometer can be used to collect High Energy Fluorescence Detected XANES (HERFD-XANES). Such experimental set-up is now available at the INE beamline [2,3] located at the synchrotron source ANKA (Karlsruhe, Germany) and application of HERFD-XANES on highly radioactive materials such as (U,Pu)O₂ is now possible.

HERFD spectra collected on stoichiometric samples

• UO₂, NpO₂, PuO₂ and AmO₂

• (U,Pu)O_2.00 prepared by powder metallurgy process [4]

Purity checked by XRD results

 single face centered cubic phase

Emission spectrometer at INE beamline (ANKA, Germany)

Conclusions

The gain in selectivity and resolution provided by HERFD at M_4,5 (3d →5f) edges of actinides (U-Am) allow to precisely probe the 5f (and 6d) occupancy experimentally. Charge transfer between

An valence shells (5f – 6d) and O 2p must be considered in FDMNES calculations to reproduce experiment. PuO₂ and AmO₂ (with 6d0_{in the ground state) experimental spectra are well}

reproduced with the following 5f configurations: Pu = 5f4.4_{and Am = 5f}5.6_{. For UO2 and NpO2 (with 6d1 in the ground state), 6d and 5f have to be taken into account in the charge transfer with O}

2p but results obtained with 6d0 and 6d1 with different 5f occupancies are not satisfactory. Calculations with an intermediate 6d occupancy are in progress. Contrary to the XANES measurements,

the substitution of U by Pu atoms is observed at both U and Pu edges in HERFD-XANES spectra collected on (U,Pu)O_2.00 samples [6]. First calculations show that charge transfer has to be

introduced in both U (5f-6d) and Pu (5f ).

Results on binary systems

Measurements on AnO

₂ Sample holder Results considering 6d1 0 20 40 60 Np-M 4 no rma lized µ (A.U.) Energy-E₀ (eV) 5f2.36d17s2 5f2.26d17s2 5f2.16d17s2 5f2.06d17s2 NpO₂

Results on

_U

_1-y

_Pu

_y

_O

₂

_{(y=0.24, 0.44 and 0.62) samples}

0 10 20 30 40 50 0.0 0.5 1.0 1.5 UO₂ NpO₂ M 4 HERFD XANE S inte nsity (A.U.)

Energy (E-E₀) (Ev)

10 20 30 40 50

0.0 0.1

Energy (E-E0) (Ev)

UO

₂

&

NpO

₂

U : [Rn] 5f

3

6d

1

s

2

|

Np

: [Rn]

5f

4

6d

1

s

2 ΔE = 1.2 eV / 1.0 eV  M₄N₆ line (M_β or 3d_3/2-4f_5/2) 0 10 20 30 40 50 0.0 0.5 1.0 1.5 PuO₂ AmO₂ M 5 HERFD XANE S inte nsity (A.U.)

Energy (E-E₀) (Ev)

10 20 30 40 50

0.0 0.1

Energy (E-E0) (Ev)

Pu

: [Rn]

5f

6

_6d

0

_s

2

_|

_Am

_{: [Rn]}

_5f

7

_6d

0

_s

2

PuO

₂

&

AmO

₂  M₅N₇ line (M_α or 3d_5/2-4f_7/2)

ΔE = 1.2 eV / 1.3 eV

PuO

₂

 Pu

+4

_{/ 5f}

4

_6d

0

Electronic configurations considered : 5f6.0_{ 5f}4.0_{/ O 2p}4.0_2p6.0

10 20 30 40 50 0.00 0.05 0.10 0.15 Pu-M 5 no rma lized µ (A.U.) Energy -E₀ (eV) PuO₂ 5f4.46d07s2 5f4.36d07s2 0 20 40 60 Pu-M 5 no rma lized µ (A.U.) Energy -E₀ (eV) 5f4.56d07s2 5f4.46d07s2 5f4.36d07s2 5f4.26d07s2 5f4.16d07s2 5f4.06d07s2 PuO₂  5f4.0  5f4.5 Pu M₅ E₀ = 3773 eV

best results = Pu 5f

4.4

 Charge transfer : Pu 5f – O 2p  +1.6 e

-

_{/ -0,8 e}

-AmO

₂

 Am

+4

_{/ 5f}

5

_6d

0 0 20 40 60 Pu-M 5 no rma lized µ (A.U.) Energy -E₀ (eV) 5f5.66d07s2 5f5.56d07s2 5f5.46d07s2 5f5.36d07s2 5f5.26d07s2 AmO₂ Am M₅ E₀ = 3890.5 eV  5f5.2  5f5.6 Electronic configurations : 5f7.0_{ 5f}5.0_{/ O 2p}4.0_2p6.0

Best results obtained for Am 5f

5.6

Charge transfer = Am 5f – O p: +1.4 e

-

/ -0,7 e

-Slight decrease of the charge transfer between Pu and Am (Δ = 0.2 e

-

₎

 in agreement with 0.14 e

-

obtained by Yu et al [5] extracted from GGA+U calculations

FDMNES calculations for elements with 6d

1

ground states

NpO

₂

 Np

+4

/ 5f

3

6d

0 0 20 40 60 Np-M 4 no rma lized µ (A.U.) Energy-E₀ (eV) 5f2.36d17s2 5f2.26d17s2 5f2.16d17s2 5f2.06d17s2 NpO₂ Np M₄ E₀ = 3842.2 eV 5f2.0  5f2.0  5f2.2

If charge transfer is only considered Np 5f - O 2p :

Experimental spectrum is not reproduced (WL & resonances)

Calculations with 6d0_(5f3.0_-5f4.0_{)  no improvement}

 Charge transfer Np 5f and 6d and O 2p

Same conclusion for UO

₂

Calculations in progress

3780 3800 3820 3840 0.0 0.2 0.4 0.6 0.8 1.0 Pu-M 5 n or ma lized µ (A.U.) Energy (eV) U_0.76Pu_0.24O_2.0 U_0.56Pu_0.44O_2.0 U_0.38Pu_0.62O_2.0 PuO₂ 3770 3775 3780 3785 3790 0.0 0.2 0.4 0.6 0.8 1.0 Pu-M 5 n or ma lized µ (A.U.) Energy (eV) U_0.76Pu_0.24O_2.0 U_0.56Pu_0.44O_2.0 U_0.38Pu_0.62O_2.0 PuO₂ WL width  3773.3(3) eV

Pu-M

₅

results

_U-M

₄

_results

3722 3724 3726 3728 3730 3732 3734 3736 0.0 0.2 0.4 0.6 0.8 1.0 U-M IV no rma lized HERFD-XANES (A.U.) Energy (eV) UO_2.00 U_0.75Pu_0.25O_2.00 U_0.54Pu_0.46O_2.00 U_0.38Pu_0.62O_2.00

• Increase of the WL width but no shift

• No shift of resonance positions

• No broadening of the WL

• Slight shift of the WL to higher energy 3790 3800 3810 3820 3830 3840 0.00 0.05 0.10 0.15 0.20 Pu-M 5 n or ma lized µ (A.U.) Energy (eV) U_0.76Pu_0.24O_2.0 U_0.56Pu_0.44O_2.0 U_0.38Pu_0.62O_2.0 PuO₂ PuO₂ MOX +20.0 +23.3 +39.0

FDMNES simulation : PuO

₂

Vs U

_0.50

Pu

_0.50

O

_2.0



Introduction of charge transfer Pu 5f – O2 p  5f

4.4

3770 3780 3790 0.0 0.2 0.4 0.6 0.8 1.0 Pu-M 5 no rma lized µ (A.U.) Energy (eV) Pu - 5f4.46d07s2 PuO₂ U_0.50Pu_0.50O_2.0 3790 3800 3810 3820 3830 3840 0.00 0.02 0.04 0.06 0.08 0.10 Pu-M 5 no rma lized µ (A.U.) Energy (eV) Pu - 5f4.46d07s2 PuO₂ U_0.50Pu_0.50O_2.0 WL width  5f band  Not OK !

Comparison

with PuO

₂ 3790 3795 3800 3805 3810 3815 3820 3825 3830 3835 3840 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 Pu-M 5 no rma lized µ (A.U.) Energy (eV) U_0.56Pu_0.44O_2.0 U_0.50Pu_0.50O_2.0 FDMNES Pu - 5f4.46d07s2 3770 3780 3790 0.0 0.2 0.4 0.6 0.8 1.0 Pu-M 5 n or ma lized µ (A.U.) Energy (eV) U_0.56Pu_0.44O_2.0 U_0.50Pu_0.50O_2.0 FDMNES Pu - 5f4.46d07s2 First results on 50% Pu

• White line width is not reproduced

• Resonance are shifted

Next step = introduction of

charge transfer U 5f-6d + Pu 5f

Finite Difference Method for Near

Edge Structure (FDMNES) code [1]

Local Density Approximation (LDA) - Green Formalism (Multiple scattering)

Input data :

• Crystallographic structure

• Cluster radius = 8 Å (143 atoms) • An and O electronic configuration

FDMNES calculations for elements with 6d

0