Supporting Information: Structural investigation of

Na3NpO4 and Na3PuO4 using X-ray di�raction and237Np Mössbauer spectroscopy

A.L. Smitha,b,∗, P.E. Raisona,∗, A. Hena, D. Bykova, E. Colineaua, J.-P.Sanchezc,d, R.J.M. Koningsa, A.K. Cheethamb

aEuropean Commission, Joint Research Centre, Institute for Transuranium Elements,

P.O. Box 2340, D-76125 Karlsruhe, Germany

bDepartment of Materials Science and Metallurgy, University of Cambridge, 27 Charles

Babbage Road, Cambridge, CB3 0FS, United Kingdom

cCEA,INAC-SPSMS, FR-38000, Grenoble, France

dUniversité de Grenoble Alpes, INAC-SPSMS, FR-38000, Grenoble, France

∗Corresponding authorEmail addresses: als77@cam.ac.uk (A.L. Smith), philippe.raison@ec.europa.eu

(P.E. Raison)

Electronic Supplementary Material (ESI) for Dalton Transactions.This journal is © The Royal Society of Chemistry 2015

1. High temperature X-ray di�raction study of α-Na3NpO4

Experimental section

α-Na3NpO4 was prepared by mixing neptunium dioxide with sodium car-bonate in a (1:2) ratio, as was done for α-Na3PuO4, and heating the mixtureunder a �ow of argon in a tubular furnace at 1093 K. The compound wasobtained with small admixtures of Na4NpO5 and unreacted NpO2.

The thermal expansion and stability of the α-Na3NpO4 phase was assessedby high temperature X-ray di�raction using a Bruker D8 X-ray di�ractometermounted with a curved Ge monochromator (111), a copper ceramic X-raytube (40 kV, 40 mA), a Vantec position sensitive detector, and equipped withan Anton Paar HTK 2000 chamber. Measurements were conducted underhelium at atmospheric pressure up to 973 K. The temperature, measuredwith a thermocouple, was previously calibrated using the thermal expansiondata of MgO [1]. The uncertainty on the temperature is estimated to be 20K at 1473 K.

Study of the compounds' thermal expansion

As the Na3AnO4 (An=Np,Pu) phase could form in Sodium-cooled FastReactors following the interaction between the sodium metallic coolant andthe (U,Np,Pu)O2 nuclear fuel, it is essential from the point of view of safetyto assess its thermal expansion behaviour.

No change was detected in the X-ray di�raction patterns recorded up to973 K except for a shift to lower 2θ values following the thermal expansionof the unit cell in all three directions. The evolution of the lattice param-eters with temperature are shown in Figure 1. The corresponding thermalexpansion coe�cients were �tted by linear regression using the following ex-pressions (1):

αa =1

a298⋅

∂a

∂T; αb =

1

b298⋅

∂b

∂T; αc =

1

c298⋅

∂c

∂T(1)

The corresponding coe�cients of thermal expansion were estimated asαa = 12.4 ⋅ 10−6K−1, αb = 34.3 ⋅ 10−6K−1, αc = 12.8 ⋅ 10−6K−1, and αvol =

60.0 ⋅ 10−6K−1 in the temperature range 298-973 K. The expansion is morelimited in the a and c directions compared to b, because of the corner- andedge-sharing of the NpO6 octahedra in the ac plane.

The latter values are larger than pure UO2, reported as αa = 10.8⋅10−6K−1

under vacuum in the temperature range 298-1600 K [2]. This con�rms that

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swelling could be signi�cant in case of a clad breach, and needs to be ac-counted for in the design of SFRs.

3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 00 . 9 9 81 . 0 0 01 . 0 0 21 . 0 0 41 . 0 0 61 . 0 0 81 . 0 1 01 . 0 1 21 . 0 1 41 . 0 1 61 . 0 1 81 . 0 2 01 . 0 2 2

(a/a 29

8, b/b

298, c/

c 298)

T e m p e r a t u r e / K

Figure 1: Evolution of the a (∎), b ( ), and c (▲) cell parameters of Na3NpO4

as a function of temperature relative to the values recorded at 298 K.

2. Na3NpO4 structure solution

After determination of the symmetry (orthorhombic) and space group(Fmmm) of the Na3NpO4 structure, and re�nement of the unit cell param-eters, the position of the heaviest atom (neptunium) was �rstly assigned tothe Wycko� position (8g), i.e. (x, 0, 0). One neptunium atom could re-produce reasonably well the experimental X-ray di�raction pattern with anarbitrary value for the fractional coordinate x=0.1. The calculated intensi-ties after re�nement of the neptunium atomic position are shown in Figure 2hereafter.

The positions of the lightest atoms - Na and O - were subsequently as-sessed by calculating 3D Fourier di�erences using the program GFourier (Ver-sion 04.06) of the Fullprof2k suite [3]. This procedure allows the visualization

3

(arbitraryunits)

Intensity(

2Θ (°)

Figure 2: Comparison between the observed (Yobs, in red) and calculated(Ycalc, in black) X-ray di�raction pattern of α-Na3NpO4 considering the nep-tunium atomic positions only. Yobs-Ycalc, in blue is the di�erence betweenthe experimental and calculated intensities. The re�nement yields Rwp =36.8, Rexp = 7.82, χ2 = 22.2. The Bragg re�ections are marked in green.Measurement at λ = Cu-Kα1.

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Na

O

O

(a) Residual density at z=0

NaO O

(b) Unit cell

Figure 3: (a) Residual electronic density (Fobs - Fcalc) calculated using theprogram GFourier at z=0 after re�nement of the X-ray di�raction data ofNa3NpO4 considering the neptunium atomic positions only.(b) Sketch of theNa3NpO4 structure (Np atoms represented in grey, Na atoms in yellow, Oatoms in red).

of the residual electronic density in the unit cell. Figure 3a shows the residualelectronic density at the height z=0. One can clearly identify the oxygen andsodium atoms in the plane (0,0,1) and determine their approximate fractionalpositions along the x and y axes. Figure 3b shows the atoms in the unit cell,stressing the correspondence between the electronic density and the atomicpositions of the light atoms at the height z=0.

In the same manner, the residual electronic density was calculated atdi�erent heights until all atoms were localized. Figure 4a is the calculationat the height z=0.5, showing the position of the oxygen and sodium atomswithin this plane.

The �nal atomic positions were �nally re�ned by the Rietveld method.Figures 5a and 5b show that there is no more residual electronic density afterre�nement of the Na3NpO4 structure considering all atomic positions, i.e. Npsite in Wycko� position (8g), Na sites in (8g), (8c) and (8f), O sites in (8i),(8d) and (16o).

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O

O

Na

Na

(a) Residual density at z=0.5

O

O

Na

Na

(b) Unit cell

Figure 4: (a) Residual electronic density (Fobs - Fcalc) calculated using theprogram GFourier at z=0.5 after re�nement of the X-ray di�raction data ofNa3NpO4 considering the neptunium atomic positions only.(b) Sketch of theNa3NpO4 structure (Np atoms represented in grey, Na atoms in yellow, Oatoms in red).

(a) z=0 (b) z=0.5

Figure 5: Residual electronic density (Fobs - Fcalc) calculated using the pro-gram GFourier at (a) z=0 and (b) z=0.5 after �nal re�nement of the X-raydi�raction data of Na3NpO4 considering all atomic positions (Np, Na and Osites).

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[1] D. Taylor, Thermal expansion data. I:Binary oxides with the sodiumchloride and wurtzite structures, MO, Brit. Ceram. Trans. J. 83 (1) (1984)5�9.

[2] A. C. Momin, E. B. Mirza, M. D. Mathews, High temperature X-raydi�ractometric studies on the lattice thermal expansion behaviour ofUO2, ThO2 and (U0.2Th0.8)O2 doped with �ssion product oxides, J. Nucl.Mater. 185 (1991) 308�310.

[3] J. Rodriguez-Carvajal, Recent advances in magnetic structure determi-nation by neutron powder di�raction, Physica B 192 (1993) 55�69.

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