Synthesis and crystal structure characterisation of sodium neptunate compounds

Journal of Nuclear Materials 413 (2011) 114–121

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Synthesis and crystal structure characterisation of sodium neptunate compounds A.L. Smith a,b, P.E. Raison a,⇑, R.J.M. Konings a a b

European Commission, JRC-Institute for Transuranium Elements, 76125 Karlsruhe, Germany Chimie-Paristech, ENSCP, 75231 Paris, France

a r t i c l e

i n f o

Article history: Received 10 September 2010 Accepted 5 April 2011 Available online 16 April 2011

a b s t r a c t The present work reports studies of the chemical reactions between neptunium dioxide and sodium oxide either in the presence of oxygen or inert gas (Ar), leading to compounds with hexavalent, heptavalent or pentavalent/tetravalent neptunium, respectively. Solid state synthesis with different NpO2/Na2O ratios led to the following polycrystalline compounds: Na2Np2O7 monoclinic (P1211), a-Na2NpO4 orthorhombic (Pbam), b-Na2NpO4 orthorhombic (Pbca), b-Na4NpO5 tetragonal (I4/mmm), Na5NpO6 monoclinic (C2/m) and a cubic compound (Fm-3m) that could either be Na3NpO4 or Na4NpO4. The crystal structures of the a-Na2NpO4 and Na2Np2O7 compounds were refined by Rietveld analysis. Evolution of the cell parameters of a-Na2NpO4 was also followed as a function of temperature up to 1273 K by X-ray diffraction. The corresponding linear thermal expansion coefficients along the different axis were determined: aa = 41.3  106 K1, ab = 35.0  106 K1, ac  0 K1. From the high temperature X-ray diffraction experiment it was also possible to evidence formation of diverse phases at different temperatures and to review parts of the Na–Np–O system. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction The main challenge for the development of innovative fast reactors fuels, envisioned in the international GEN-IV program, comes from the objective to incorporate minor actinides (Np, Am, Cm) into the fuels up to significant concentrations. Though various types of fuels, e.g. nitrides, carbides, metals, are potential candidates, (U, Pu)O2 mixed oxide fuels are currently considered as reference for Sodium-cooled Fast Reactors (SFRs) for which substantial experience has already been accumulated in terms of fabrication, reactor operation, reprocessing and risk assessment. However, (U, Pu)O2 mixed oxides have also various drawbacks: for instance, in case of a clad breach, though extremely rare during normal operation conditions, sodium will enter the fuel pin and react with the fuel. Admixing minor actinides (Np, Am, Cm) to (U, Pu)O2 will introduce a more complex chemistry for which many data are still missing. In that respect a scientific program has been started at the institute for transuranium elements to assess the aftermaths on the safety when minor-actinides-bearing-fuels come into contact with the metallic coolant. This paper reports some preliminary results. The phase relations in the (Na–U–O) system and the thermochemical properties of the various phases have already been studied in the past by several workers. In particular, the following sodium uranates in which the uranium has either the valency of VI or V were described intensively by Cordfunke and co-workers [1–3]: Na2U2O7, Na2UO4 (a and b), Na4UO5, NaUO3 and Na3UO4. ⇑ Corresponding author. E-mail address: [email protected] (P.E. Raison). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.04.009

In the 1980s Mignanelli and Potter [4–6] studied the reaction between liquid sodium and urania, urania–plutonia solid solutions and urania–plutonia solid solutions containing cations to simulate fission products. It was then evidenced that in the temperature range of the fuel during operation, close to the pellet rim, (923 K), the main reaction products were Na3MO4 where M = U, Pu, U1xPux. These compounds were found to be of lower density and with less than half the thermal conductivity relative to the mixed oxide [7–9], which would lead to local swelling and temperature increase. The authors also measured for different ternary and quaternary systems the thermodynamic potential or partial molar Gibbs energies of oxygen (DGeq O2 ) which can also be expressed as an oxygen concentration C eq O in the liquid sodium phase. They showed that the metallic coolant coming into contact with the urania–plutonia solid solution could lead to an oxygen concentration increase in the liquid sodium in conjunction with the reduction to a lower valency of the plutonium in the oxide phase. Concerning the interaction of minor actinide oxides with alkali metals, Keller and his corworkers appear as the pioneers of such studies. The authors first described [10] the thermal, hydrolytic and structural properties of ternary oxides containing hexavalent transuranium elements and compared them to the corresponding uranates (VI). Depending on the experimental conditions, they obtained the following compounds: Na2Np2O7, a-Na2NpO4, Li4AnO5 (An = Np, Pu, Am), a-Na4AnO5 (An = U, Np, Pu, Am), b-Na4AnO5 (An = Np, Pu), Li6AnO6 (An = Np, Pu, Am), and Na6AnO6 (An = Np, Pu, Am) and found them to be isostructural with the uranates of the corresponding formula. They also presented results on the chemical and structural properties of alkali-transuranates

A.L. Smith et al. / Journal of Nuclear Materials 413 (2011) 114–121

containing pentavalent transuranium elements [11]: Li3AnO4 (An = Np, Pu, Am), Na3AnO4 (An = Np, Pu, Am) and Li7AnO6 (An = Np, Pu, Am). The synthesis and characterization of the structural, thermodynamic, magnetic and spectroscopic properties of these ternary and other complex actinide mixed oxides were later reviewed by Keller et al. in 1972 [12] and Morss in 1982 [13] and 1994 [14]. In the latter work, Morss pointed out a need for reexamination of the sodium uranates and neptunates and the lack of structural and thermodynamic characterization. The present work was focused on the reaction products formed under various experimental conditions between neptunium dioxide and sodium oxide and re-examined the Na–Np–O system. 2. Experimental 2.1. Raw materials Compounds were prepared by grinding together accurately weighted samples of neptunium dioxide (237NpO2 from ORNL, Oak Ridge) and sodium oxide (Na2O Alfa Aesar). The sodium oxide was stored in an argon filled dry box in order to avoid formation of NaOH or Na2CO3. Thermogravimetric measurements and differential thermal analysis showed that Na2O contained about 12 wt% of Na2O2. Furthermore, because of the oxidation and vaporization of sodium oxide upon heating in oxygen, all mixtures were prepared with an excess of 20 wt% of sodium oxide. The mixtures were placed into alumina crucibles and heated in a flow of pure oxygen. Using an oxidizing gas was necessary to stabilize neptunium in its higher oxidation states VI and VII. However, one experiment was conducted under a flow of argon in a nickel crucible leading to a compound with pentavalent/tetravalent neptunium. Because of the significant gamma dose rate of neptunium decay products (233Pa), each experiment was carried out with no more than 120 mg of neptunium oxide. The temperature range explored in the present work is comprised between 673 and 1273 K. 2.2. X-ray diffraction The structure of the products was determined at room temperature by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer mounted in a Bragg–Brentano configuration with a curved Ge monochromator (1, 1, 1), a ceramic copper tube (40 kV, 40 mA) equipped with a LinxEye position sensitive detector. The data were collected by step scanning in the angle range 10° 6 2h 6 120° at a 2h step size of 0.0092°. For the measurement, the powder was deposited on a silicon wafer to minimize the background and dispersed on the surface with 2 or 3 drops of isopropanol. Structural analysis were performed by the Rietveld method with the Fullprof2k suite [15]. The thermal stability of the a-Na2NpO4 compound was also followed by high temperature X-ray diffraction experiments. The data were collected on a Bruker D8 X-ray diffractometer mounted with a curved Ge monochromator (1, 1, 1), a copper ceramic X-ray tube (40 kV, 40 mA), a Vantec position sensitive detector and equipped with an Anton Paar HTK 2000 chamber. Measurements were conducted up to 1273 K under helium, in the angle range 16° 6 2h 6 90° with a 2h step size of 0.017°. From those data, some phase transitions were characterized and the material’s thermal expansion coefficients calculated.

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ultrathin coating of gold deposited on the sample by low vacuum sputter coating with a BAL-TEC SCD005 apparatus. 3. Results and discussion As already mentioned, Keller et al. [10] reported the reactions occurring in the Na–Np–O system for selected NpO2–Na2O ratios. A similar approach was adopted in the present studies and the structural parameters determined for each compound are described here after. 3.1. Na2NpO4 Na2NpO4 was obtained by grinding together the neptunium and sodium oxide powders in a (NpO2:Na2O) = (1:1.2) ratio and heating the mixture twice at 673 K for 12 h in pure oxygen with a regrinding step in between the two thermal treatments. The reaction product was examined by scanning electron microscopy: it consists of small crystallites (500 nm in size or less) that are agglomerated (average size: 10 lm). The experimental X-ray diffraction pattern (Fig. 1) revealed that a-Na2NpO4 is isostructural to a-Na2UO4 and therefore can be indexed with an orthorhombic unit cell with the space group Pbam, but not in the Cmmm space group as initially reported by Keller et al. Several Bragg reflections could not be indexed in the latter. The Rietveld analysis revealed that the sixfold coordinated neptunium atoms are bonded to oxygens with minimum and maximum lengths 0.1762(5) nm and 0.2086(5) nm respectively, whereas the sodium atoms are coordinated to seven oxygens, with minimum and maximum bond lengths 0.2483(5) nm and 0.2731(5) nm. Those values are consistent with the ones determined for the isostructural uranate compound by neutron diffraction [16]. The a-Na2NpO4 structure presents endless chains of edge-sharing NpO6 octahedra along the c axis (Fig. 2). The structural parameters determined in the present work are summarized in Tables 1 and 2. The cell parameters are in good agreement with the ones published by Keller et al. 3.2. Na4NpO5 To prepare the Na4NpO5 compound, NpO2 and Na2O were ground in a (1:2.4) ratio and heated for 24 h at 673 K in pure oxygen, reground and heated again for 12 h at the same temperature. This synthesis led to the formation of the b form of Na4NpO5, which is tetragonal. Keller et al. [10,12] reported a cubic form (a = 0.4739 nm) which was designated by a and supposed to transform to the b form upon heating above 773 K. These results could not be confirmed in the present experiments. The X-ray diffraction pattern (Fig. 3) clearly showed the peaks of a body centred unit cell (space group I4/mmm) assigned to the b form. Similar to the corresponding uranium compound [13], vertices-sharing NpO6 octahedra form chains along the c-direction (Fig. 4). Unfortunately, the quality of the data was not good enough to perform a complete Rietveld refinement and release the atomic positions. The structure of the uranium compound was used as model. Although Morss reported for the uranium compound an NaCl structure with (4Na + U) in disordered cations sites [13], more recent experiments on the plutonium compound reported the existence of the alpha form to be uncertain [17], which appears to be also the case for the neptunium compound.

2.3. Scanning electron microscopy 3.3. Na2Np2O7 Some samples were examined by scanning electron microscopy (SEM) with a FEI (Philips) XL 40 apparatus with a 3 nm resolution. As the compounds were nonconductive, they were coated with an

Two kinds of experiments led to the formation of Na2Np2O7. Firstly, it was observed as a secondary phase while mixing

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Fig. 1. Comparison between the observed (Yobs, in red) and calculated (Ycalc, in black) X-ray diffraction pattern of a-Na2NpO4. Yobs–Ycalc, in blue is the difference between the experimental and calculated intensities. The Bragg reflections are marked in green.

Fig. 2. Representation of the structure of a-Na2NpO4.

neptunium dioxide and sodium oxide in a (1:1.2) ratio and heating the mixture for 12 h under pure oxygen at successively 973 K and 1123 K. The reaction product was then analysed by scanning electron microscopy showing two different grain morphologies (Fig. 5). The synthesis led to the formation of the a-Na2NpO4 compound, described previously, and to a Na2Np2O7 compound with a monoclinic unit cell (Figs. 6 and 7). A simultaneous Rietveld refinement of the two phases led to the same parameters for Na2NpO4 (Tables 1 and 2) and to the parameters reported in Tables 1 and 3 for Na2Np2O7. This monoclinic description differs from the rhombohedric unit cell reported in [10] and from the orthorhombic one reported in [12].

A second experiment was carried out which was an exact duplication of Keller’s work. It consisted of weighing accurately the starting materials in a (NpO2:Na2O) = (2:1) ratio, grinding them together and heating them twice at 873 K for 12 h in O2. The results could not confirm the rhombohedric phase. Though the X-ray diagram revealed a poorly crystallized sample, the Bragg reflections were attributed to a monoclinic unit cell. Concerning the uranium phases, Morss [13] pointed out that the structure reported in 1972 by Kovba [18] for Na2U2O7 was monoclinic. Cordfunke et al. [1] also described a monoclinic C-face centred subcell with the following parameters: a = 0.6895 nm, b = 0.3916 nm, c = 0.6400 nm and b = 111.47°. Their results differed

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A.L. Smith et al. / Journal of Nuclear Materials 413 (2011) 114–121 Table 1 Summary of the compounds observed in the present studies and their corresponding structural parameters. Compound

a-Na2NpO4

b-Na2NpO4

c-Na2NpO4

Na2Np2O7

b-Na4NpO5

Na5NpO6

Temperature Symmetry Z Space group a (nm) b (nm) c (nm)

RT Orthorhombic 2 Pbam (55) 0.9715(3) 0.5732(3) 0.3459(3) 90 90 90 0.19259(1)

RT (after heating at 1273 K) Orthorhombic 4 Pbca (61) 0.5782(5) 0.5930(5) 1.1649(5) 90 90 90 0.3994(1)

1273 K Tetragonal 4 ? 0.6018(5) 0.6018(5) 1.1980(5) 90 90 90 0.4339(1)

RT Monoclinic 2 P21 (4) 0.6830(3) 0.7809(3) 0.6317(3) 90 111.28(1) 90 0.31392(1)

RT Tetragonal 2 I4/mmm (87) 0.7530(5) 0.7530(5) 0.4619(5) 90 90 90 0.2619(1)

RT Monoclinic 2 C2/m (12) 0.581(5) 0.998(5) 0.574(5) 90 110.66(5) 90 0.311(1)

a b

c Cell volume V (nm3)

Table 2 Refined atomic positions in the a-Na2NpO4 compound. The atomic thermal parameters are taken from [16]. Rwp = 14.2 (standard deviation = 3r). Atom

Ox. state

Wyckoff

x

y

z

B0 (nm2)

4h 2a 4g 4h

0.1838(4) 0 0.1645(7) 0.0533(7)

0.4384(6) 0 0.1293(11) 0.1824(10)

0.5 0 0 0.5

0.0113 0.0036 0.0076 0.0078

a-Na2NpO4 Na Np O1 O2

+1 +6 2 2

also from those of Kovba in 1958 who described a rhombohedric form [19] and those of Carnall et al. who suggested an orthorhombic form [20]. The other alkali metal diuranates and dineptunates, i.e. K2U2O7, Rb2U2O7, K2Np2O7 and Rb2Np2O7 were first reported to have a rhombohedric structure related to CaUO4 [21]. However, an inconsistency remained with only 1.5 formula units per cell. Later, Van Egmond and Cordfunke [22] described the structure of K2U2O7 and Rb2U2O7 as monoclinic with two formula per unit cell, which was confirmed later by Saine’s work on single crystal [23]. No reexamination of the dineptunates has been carried out so far. From the present work, only the monoclinic structure of Na2Np2O7 could be confirmed in which the sixfold coordinated neptunium atoms are bonded to oxygens with minimum and maximum lengths 0.164(2) nm and 0.255(2) nm, respectively and the sodium atoms are coordinated to seven oxygens, with minimum and maximum bond lengths 0.230(2) nm and 0.276(2) nm, respectively. Those

values are comparable to the ones determined from a structural refinement on a K2U2O7 single crystal [23] (U–O = 0.185– 0.289 nm, K–O = 0.261–0.274 nm).

3.4. Na5NpO6 Keller et al. reported [12] that a (NpO2:Na2O) = (1:3) ratio leads to the formation of a-Na4NpO5 at 673 K and that a further treatment at 773 K leads to the formation of hexagonal Na6NpO6. The authors described the structure to be isostructural to Li6ReO6 (P3112). In order to synthesise this compound, the NpO2:Na2O powders were mixed together in a (1:3.6) ratio and the mixture heated at 673 K for 12 h and then twice at 773 K for 12 h with X-ray analysis and regrinding between each thermal treatment. The same phase was obtained after each thermal treatment. The X-ray data analysis showed that the obtained compound crystallyzes in the monoclinic phase with the space group C2/m. The unit cell parameters are reported in Table 1. The reaction product appears isostructural with the monoclinic Li5NpO6 compound reported by Morss [14]. For this composition, neptunium is expected to be heptavalent. Because of the poor crystallinity of the compound the structure of Na5NpO6 could not be refined by Rietveld analysis in the present studies, only the monoclinic phase and the space group could be confirmed. The reported a-Na4NpO5 cubic compound was not observed after the treatment at 673 K.

Fig. 3. X-ray diffraction pattern after Rietveld analysis of the b-Na4NpO5 compound.

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b c a

Fig. 4. Representation of the structure of b-Na4NpO5.

(Fm-3m), with a cell parameter a = 0.477 nm. The compound so formed appeared isotypic with the uranium compound Na3UO4 described in reference [25]. However, Pillon et al. reported in [26] that formation of Na3UO4 is not direct and that an intermediate compound of formula Na4UO4 where uranium is tetravalent forms instead. This latter has a similar crystal structure and almost identical cell parameter. From the present experiment it was not possible to conclude whether under argon and at this relatively low temperature the pentavalent neptunium compound Na3NpO4 really formed or if the Na4NpO4 with a tetravalent neptunium formed. Further work is needed to determine the exact composition observed under those experimental conditions. 3.6. High temperature X-ray diffraction experiment on the a-Na2NpO4 compound

Fig. 5. SEM picture of the reaction product obtained with a (NpO2:Na2O) = (1:1.2) ratio and heating at 973 K and 1123 K. (a) Na2NpO4; (b) Na2Np2O7.

3.5. Experiment under inert atmosphere One of the experiment was performed under inert atmosphere. Neptunium and sodium oxide powders were mixed and ground together in a (NpO2:Na2O) = (1:1.2) ratio and heated at 673 K for 12 h under argon in a nickel crucible. For this specific composition, a rhombohedric or a monoclinic compound of formula Na2NpO3 was expected where the neptunium is tetravalent, as observed for the plutonium [17] and cerium [24] compounds, respectively. Instead, the X-ray diffraction pattern showed a mixture of unreacted NpO2 with another phase, identidied as face-centered cubic

High temperature X-ray diffraction experiments were carried out at successively room temperature, 523, 673, 773, 873, 973, 1073, 1173, 1273 K and room temperature again, under He, in order to better understand the a–b transformation of the Na2NpO4 compound reported to happen at 1073 K [10]. Cordfunke et al. [1] who examined the a–b phase transition in the case of the sodium uranates using a high-temperature Guinier camera, determined the transition temperature to be about 1193 K and reversible. The structure of the b-Na2UO4 compound was reported [27] to be orthorhombic, with the space group Pbca, and the following cell parameters a = 0.58079 nm, b = 0.59753 nm, c = 1.17179 nm. The phenomena observed while heating the a-Na2NpO4 compound were actually quite complex. It appeared firstly that aNa2NpO4 partially decomposed between 673 K and 773 K into Na2Np2O7 monoclinic and another unidentified phase. At 873 K though, only the orthorhombic a-Na2NpO4 and monoclinic Na2Np2O7 phases were observed. At 1073 K NpO2 started to appear. At 1173 K, Na2Np2O7 monoclinic disappeared and a new phase for

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Fig. 6. Comparison between the observed (Yobs, in red) and calculated (Ycalc, in black) X-ray diffraction pattern of a-Na2NpO4 + Na2Np2O7. Yobs–Ycalc, in blue is the difference between the experimental and calculated intensities. The Bragg positions are marked in green. Upper Na2NpO4 and lower Na2Np2O7. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 3 Refined atomic positions in the Na2Np2O7 compound. Rwp = 13.5. (standard deviation = 3r). Atom

Ox. state

Na2Np2O7 Np1 +1 Np2 +1 Na1 +6 Na2 +6 O1 2 O2 2 O3 2 O4 2 O5 2 O6 2 O7 2

Wyckoff

x

y

z

B0 (nm2)

2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a

0.016(2) 0.511(3) 0.497(3) 0.007(3) 0.129(3) 0.35(1) 0.072(3) 0.380(3) 0.567(3) 0.121(3) 0.283(3)

0.5 0.244(3) 0.025(3) 0.763(3) 0.245(3) 0.515(3) 0.009(3) 0.262(3) 0.232(3) 0.509(3) 0.917(3)

0.004(3) 0.004(4) 0.513(3) 0.494(3) 0.972(3) 0.024(3) 0.274(3) 0.710(3) 0.291(3) 0.300(3) 0.005(3)

0.005 0.005 0.001 0.001 0.012 0.012 0.012 0.012 0.012 0.012 0.02

The decomposition of the Na2NpO4 compound might occur according to reaction (1):

2Na2 NpO4 ðsolidÞ ! Na2 Np2 O7 ðsolidÞ þ Na2 OðsolidÞ

ð1Þ

Then, between 873 K and 1073 K, Na2Np2O7 decomposes according to reaction (2) and NpO2 appears.

Na2 Np2 O7 ðsolidÞ ! 2NpO2 ðsolidÞ þ Na2 OðsolidÞ þ O2 ðgasÞ Fig. 7. Representation of the structure of the Na2Np2O7 compound.

which it was not possible to fit any known model appeared. At 1273 K, the Bragg reflections revealed again the existence of new phases: the a-Na2NpO4 and NpO2 compounds were still present but one could also note the formation of a tetragonal c-Na2NpO4 phase and of the tetragonal b-Na4NpO5 phase. After cooling at room temperature, the X-ray diffraction pattern finally showed a mixture of b-Na2NpO4 orthorhombic, b-Na4NpO5 and NpO2. The cell parameters for the phases present at 1273 K and after cooling at room temperature are reported in Table 1.

ð2Þ

As already mentioned, at 1173 K, Na2Np2O7 completely disappeared and a new intermediate phase that could not be identified appeared. At 1273 K the latter was not present but the b-Na4NpO5 phase was observed which was not expected to form at this high temperature and under those conditions. Two hypotheses regarding its formation can be proposed: Hypothesis 1. Solid state reaction between NpO2 and Na2O leading to Na4NpO5. One possible hypothesis to explain the formation of the Na4NpO5 compound would be a solid state reaction between NpO2 and Na2O. Combining (1) and (2), reaction (3) gives:

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2Na2 NpO4 ðsolidÞ ! 2NpO2 ðsolidÞ þ 2Na2 OðsolidÞ þ O2 ðgasÞ

ð3Þ

Hypotheses 1 and 2 seem plausible. However, if the second one can explain the mechanisms in terms of displacement of equilibriums related to the vaporization of the sodium oxide, the insertion of two different volatile species in the matrix of Na2NpO4 seems nonetheless a complex process and doubtful considering the low partial pressure of the volatile species. Hypothesis 1 should be then favoured. In conclusion to this paragraph, the phase transition of the a-Na2NpO4 compound occurs between 1073 K and 1273 K. This transition takes place in favour of the formation of a tetragonal phase that can be referred as c. The b form of the compound, orthorhombic, for which the space group was found to be Pbca and not Fmmm as described by Keller, was only observed after cooling down to room temperature. This suggests that the tetragonal phase c is the high temperature form of Na2NpO4. This experiment also showed that the cell parameters of the aNa2NpO4 form increase linearly along the a- and b-directions as a function of the temperature, but remained fairly constant along the c-direction. The corresponding linear thermal expansion coefficients along the a and b axes were calculated and can be expressed by the following equations:

It can be envisioned that the reaction (4) with a (NpO2:Na2O) = (1:2) ratio would occur:

1 NpO2 ðsolidÞ þ 2Na2 OðsolidÞ þ O2 ðgasÞ ! Na4 NpO5 ðsolidÞ 2

ð4Þ

An excess of NpO2 remains though, which is why NpO2 is observed also at 1273 K. The only drawback for Hypothesis 1 is the fact that the b-Na4NpO5 phase was not observed at lower tyemperature. Already at 1073 K, when a-Na2NpO4, Na2Np2O7 and NpO2 are present, it would be expected that Na4NpO5 forms as a minority phase. Hypothesis 2. Insertion of the gaseous species Na(g) and NaO2(g) into the matrix of the Na2NpO4 compound. Previously to the experiments described in this paper, a study of the vaporization behaviour of sodium oxide upon heating under a gas flow of argon and air has been carried out in house. It appeared that Na2O(solid) could vaporize according to reaction (5) above 1253 K under argon and above 1168 K under air. The high temperature X-ray diffraction experiment has been carried out under helium but it is probable that sodium oxide starts to vaporize in the same range of temperature, i.e. 1123–1273 K.

2Na2 OðsolidÞ ! 3NaðgÞ þ NaO2 ðgÞ

aa ðTÞ ¼ 1=a298  @a=@T and ab ðTÞ ¼ 1=b298  @b=@T

ð5Þ

. In the temperature range 298–1173 K they were found to be equal to aa (T) = 41.3  106 K1 and ab (T) = 35.0  106 K1 which are relatively large values in comparison to other oxides, usually in the range of 10  106 K1.

Insertion of the gaseous species Na(g) and O2(g) in the matrix of the Na2NpO4 compound leading to Na4NpO5 according to reaction (6) is envisageable

1 Na2 NpO4 ðsolidÞ þ 2NaðgasÞ þ O2 ðgasÞ ! Na4 NpO5 ðsolidÞ 2

ð6Þ 3.7. The Na–Np–O phase diagram

The vaporization of Na2O (solid) in the temperature range 1123–1273 K displaces the equilibrium of both reactions (1) and (2) to the right. This would explain why Na2Np2O7 is totally absent in the X-ray diffraction pattern at 1173 K and above.

From the present study, a preliminary sketch of the ternary Na–Np–O phase diagram between 673 K and 1273 K, covering a large range of oxidation potential, can be drawn, as shown in

O 0.0

1.0

0.1

0.9

0.2

0.3

0.4

0.6

Na4NpO5 Na2O2 0.5

Na2NpO3

0.6

Na3NpO4

0.7

O

0.5

at.

0.7

Na2NpO4

%

%

NpO2

0.8

Na2Np2O7

at.

Np

(Np2O5)

Na5NpO6 0.4

Na4NpO4

Na2O

0.3

Na6NpO5

0.8

0.2

0.9

0.1

1.0

Np

0.0

0.0 0.1

0.2

0.3

0.4

0.5

0.6

at % Na

0.7

0.8

0.9

1.0

Na

Fig. 8. Preliminary sketch of the Na–Np–O phase diagram between 673 and 1273 K, covering a large range of oxidation potential. The compounds marked in red have not been identified in the present studies but should exist. Np2O5 [28] exists and is mentioned here as an indication of the possible pentavalent state of neptunium. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

A.L. Smith et al. / Journal of Nuclear Materials 413 (2011) 114–121

Fig. 8. Further studies are required to determine the coexisting phases and confirm the oxidation state of the neptunium in each compound. Future work will address those issues as well as dedicated studies of compounds with neptunium in hepta-, pentaand tetravalent oxidation state. 4. Summary The present work has reexamined the Na–Np–O system and several crystallographic structures have been reported. It was confirmed that a-Na2NpO4 is orthorhombic and belongs to the space group Pbam. Its structure has been refined and is isostructural to the uranium analogue. High temperature X-ray measurements showed that a tetragonal form of Na2NpO4, denoted as c, forms above 1173 K and that the orthorhombic b-form of the compound, belonging to the space group Pbca, is found after cooling at room temperature. Na2Np2O7 was found to be monoclinic with the space group P1211 and b-Na4NpO5 tetragonal with space group I4/mmm. The existence of the Na2Np2O7 rhombohedric and a-Na4NpO5 compounds seems uncertain and needs to be addressed in more detail in future studies. The Na6NpO6 hexagonal phase was not confirmed but instead a monoclinic Na5NpO6 compound was obtained (space group C2/m) in which neptunium is expected to be heptavalent. Acknowledgements The authors would like to express their gratitude to H. Thiele and B. Cremer for conducting the SEM analysis and to D. Bouexière, G. Pagliosa and R. Eloirdi for the collection of the X-ray data. A.S. would also like to thank Prof. G. Cote from Chimie-Paristech for his strong and continuous support and the European Commission traineeship programme.

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