Insights into the fabrication and structure of plutonium pyrochlores

Experimental & theoretical insights into the structural response of complex oxides to dopant elements – the example of plutonium within pyrochlore.


Introduction
Due to their unique properties the class of oxides with the pyrochlore crystal structure have been studied for many different applications. These include thermal barrier coatings, 1 solid-oxide fuel cells, 2 and nuclear waste forms for the immobilisation of radionuclides. 3 They are of particular interest for the immobilisation of actinides such as plutonium due to their high chemical exibility, high tolerance to radiation damage, and high aqueous durability. [4][5][6] In addition to being a key phase in SYNROC multiphase ceramics, 6 pyrochlores have also been studied as single phase waste forms to immobilise actinide rich waste-streams such as separated plutonium from spent nuclear fuel reprocessing. 7 The pyrochlore crystal structure is a superstructure of the uorite structure and its general formula can be described as A 2 B 2 X 6 Y. Most commonly pyrochlores are of the (3+, 4+) type with rare earth-elements favouring the A-site and 3d, 4d, or 5d transition metals favouring the B-site. 8 (2+, 5+) type pyrochlores can also form but have not been widely studied for application as waste-forms. If the origin of the cubic (Fd 3m) structure is chosen to coincide with the B-site, the A cations occupy the 16d, the B cations the 16c, and the oxygen anions on the X and Y sites the 48f and 8b Wyckoff positions, respectively. In contrast to simple uorite structured compounds such as ZrO 2 , pyrochlores contain combinations of aliovalent cations and the charge compensation required to maintain electrical neutrality occurs via the introduction of oxygen vacancies. Pyrochlore structured compounds are missing one eighth of the anions and the resulting oxygen vacancy is localised at the 8a Wyckoff position. The pyrochlore structure can be viewed as being ordered with respect to the uorite structure. The A-site cations sit in the center of a scalenohedron and are coordinated to six X anions and two Y anions where the cation-anion bond length is slightly shorter. The B-site cations are coordinated solely to six X anions which form a trigonal antiprism around the site. The oxygen deviation parameter x is an indicator of the distortion of the coordination polyhedra surrounding the A and B cations. If x equals 0.375 the A site cations are surrounded by anions arranged as a regular cube and the ideal uorite structure is observed. For x ¼ 0.3125 the B site anion arrangement is regular octahedral which corresponds to the ideal pyrochlore crystal structure. 4,9 The pyrochlore structure has been shown to be exible and will accommodate cations with a variety of ionic radii at the A-and B-sites. Pyrochlores of mixed cationic occupancy can also be formed and these are referred to as non-stoichiometric pyrochlores. 10,11 Due to the dismantling of nuclear weapon programmes, many countries have large volumes of separated plutonium 12 which may ultimately require immobilisation in a ceramic wasteform. We focus in this study on zirconate pyrochlores because they are known to retain their crystalline structure when subjected to radiation damage; self-irradiation in zirconate pyrochlores leads to the formation of a defective uorite structure whereas titanate pyrochlore analogues readily amorphise. 13 Moreover, ZrO 2 -based pyrochlores show high aqueous durability 14,15 which is another important property for nuclear waste forms. In order to benet from the inherent advantages of such a crystalline waste form, structural uptake of the radionuclide has to be ensured. Previous studies have probed the uptake of curium at ppm levels on the A site of a zirconate pyrochlore 16 however, to investigate the ability of pyrochlore to serve as potential wasteform for actinides it is important to synthesize materials where the actinide is a major component. Icenhower et al. performed dissolution experiments on titanate pyrochlores containing about 12 wt% PuO 2 , 5 Nästren et al. 17 synthesised Nd 1.8 An 0.2 Zr 2 O 7 (where An ¼ Pu, Am, Np, Th, U) pyrochlores by a liquid inltration method into porous ceramic beads, and Gregg et al. 18 used a modied alkoxide nitrate route to fabricate lanthanum zirconate pyrochlores. To the best of our knowledge, this is the rst time a hydroxide co-precipitation route, for the formation of Pupyrochlores, has been reported in the literature. A wetchemical co-precipitation route was adapted with the aim of ensuring homogeneous incorporation of Pu within the pyrochlore crystal structure. Samples doped with 5 and 10 mol% Pu were fabricated and the structural uptake was probed by complimentary experimental techniques (XRD, SEM, and XAS) supported by ab initio structural calculations. The data presented here provides new insights into the preferred mechanism of incorporation of Pu within the pyrochlore crystal structure.
Pyrochlore compounds have been extensively investigated by computational methods. 10,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Radiation induced effects such as damage cascade formation, [21][22][23] amorphization, 30 the energetics of defects formation, 19,[24][25][26][27][28][29]34 and order/disorder transitions 10,[20][21][22][23]34,35 have been studied, and the results used to support the interpretation of experimental results. Cleave et al. 31 and Williford & Weber 32 investigated the incorporation of Pu into a range of rare earth pyrochlore compounds. On the basis of computed solution energies they predicted the incorporation of Pu(III) onto the Nd-site in Nd 2 Zr 2 O 7 as the most energetically preferable mechanism. We have directly tested this prediction in our studies. Recently, Perriot et al. 33 have considered the behaviour of Pu(III) and Pu(IV) across pyrochlore-defect uorite structural interfaces and predicted the preferential segregation of Pu(IV) into the disordered phase. It should be noted that all these studies were performed using a force-eld approach with Buckingham-type interaction potentials between atoms. Li et al. 34,36 have shown that, although such a force-eld based approach oen results in reasonable predictions, it can produce defect formation energies that are signicantly different from those calculated from ab initio methods. Recently, Finkeldei et al. 10 have shown that the formation enthalpies and structure of Nd x Zr 1Àx O 2À0.5x pyrochlores can be accurately simulated by density functional theory (DFT)-based ab initio methods. Here we use the same approach to support the reported experimental results with the computation of Pu solution energies in Nd 2 Zr 2 O 7 pyrochlore.

Fabrication of plutonium pyrochlore ceramics
The synthesis work described below was carried out in dedicated glovebox lines designed for the handling of alphaemitting radioisotopes such as plutonium. All samples were prepared by a wet chemical precipitation or co-precipitation approach to obtain highly homogeneous ceramic powders. Aqueous standard solutions of 0.3 mol L À1 were prepared from ZrOCl 2 $8H 2 O and Nd(NO 3 ) 3 $6H 2 O dissolved in de-ionised water. The concentrations of the solutions were checked by inductively coupled plasma optical emission spectrometry (ICP-OES). The precipitation of the hydroxides took place in a glovebox line with an air atmosphere, whereas the thermal treatment was carried out in a furnace in a glovebox line with an argon atmosphere. The Pu precursor used was plutonium nitrate ( 239 Pu) which had been previously characterised by thermal analysis. The weight loss observed occurred in two steps which is in agreement with the reported thermal decomposition of Pu(NO 3 ) 4 $xH 2 O (x ¼ 3, 5) to PuO 2 , via a plutonylnitrate intermediate. 37,38 The nitrate was found to be X-ray amorphous and on the basis of the former analysis assumed to have a stoichiometry of Pu(NO 3 ) 4 $5H 2 O. 39 Precautions such as acidication of the solutions and direct processing were taken to avoid the formation of a plutonium polymer.
Slight modications of the Pu-pyrochlore fabrication process led to two different synthesis procedures which are hereaer referred to as synthesis route 1 and synthesis route 2. To obtain highly homogeneous materials, the synthesis of 239 Pupyrochlore was carried out by a co-precipitation route which was developed by the authors. 40 A stoichiometric Nd 2 Zr 2 O 7 pyrochlore was synthesised at the same time to serve as a reference. To synthesise 4 mmol Nd 2 Zr 2 O 7 , 8 mmol of aqueous Nd(NO 3 ) 3 and ZrOCl 2 solutions (diluted to a concentration of 0.1 mol L À1 ) were mixed and transferred (1-3 drops per s) into 100 mL of conc. NH 4 OH solution to co-precipitate the hydroxides. The precipitate was separated from the supernatant by centrifuging (4000 rpm, 10 min) and the supernatant was subsequently checked for complete precipitation. Six washing steps were performed on the precipitate, each using 250 mL of deionised water, until the washing water was free of chloride and nitrate. The absence of chlorine and nitrates was conrmed using dipsticks (Macherey-Nagel). The precipitate was transferred into the inert gas glovebox line and dried at 60 C. Aer grinding, a calcination step was performed at 600 C for 2 h with a heating and cooling rate of 4.8 C min À1 . Aer a second grinding step two pellets were pressed in a 6 mm die at 707 MPa. The pellets were sintered under reducing atmosphere (Ar-5% H 2 ) at 1450 C for 10 h. A heating and cooling rate of 200 C h À1 was used. One of the pellets was subsequently crushed for analysis by XRD.
Synthesis route 1. The appropriate amounts of Nd(NO 3 ) 3 and ZrOCl 2 standard solution (Table 1) were mixed and diluted to a concentration of 0.1 mol L À1 with deionised water, in a PE bottle (V ¼ 100 mL). The desired amount of Pu(NO 3 ) 4 $5H 2 O was dissolved in 1 mol L À1 HNO 3 to yield a 0.1 mol L À1 Pu-nitrate solution. First, the Pu-hydroxide was precipitated dropwise in 40 mL liquid ammonia with the help of an Eppendorf pipette. Aerwards, Nd(OH) 3 and Zr(OH) 4 were co-precipitated by adding the solution via a dropping funnel (1 drop per s) to the liquid ammonia which already contained the precipitated Puhydroxide. The sample was then washed using the same procedure described above for the Nd 2 Zr 2 O 7 reference material. The calcination and sintering steps were performed under reducing atmosphere (Ar-5% H 2 ). The pellets were pressed with a 6 mm die at 760 MPa. The sintering parameters are given in Table 2.
Synthesis route 2. In order to further improve the homogeneity of the nal product all the metal cations were precipitated simultaneously. The aqueous solutions of Nd(NO 3 ) 3 and ZrOCl 2 were acidied to c(HNO 3 ) ¼ 1 mol L À1 and mixed prior to the addition of the Pu solution to avoid the formation of a Pu polymer. The desired amount of Pu(NO 3 ) 4 $5H 2 O (Table 1) was dissolved in nitric acid. At the same time the Pu concentration was reduced to c(Pu) ¼ 0.01 mol L À1 while the acidity of the nitric acid was kept constant at 1 mol L À1 . The light green solution was added slowly to the acidied metal salt solution which was shaken. The mixed metal solution was added into 40 mL concentrated ammonia solution via a dropping funnel (1 drop per s). The precipitate was separated using a centrifuge, washed by stirring with deionised water, and dried as described above. Two pellets were pressed, the rst was for SEM and XRD characterisation and the second was taken for the XAS measurement. The thermal treatment was carried out according to Table 2.

Characterisation
The equipment used for the characterisation of the pyrochlores is specied below.
2.2.1 XRD, SEM and EDX. All samples were characterised by XRD using a D2 Phaser (Bruker) diffractometer. The pellets were thoroughly ground prior to measurement. The patterns were recorded from 10-90 2q with a step size of 0.01 and an acquisition time of 1 s per step. Subsequent to the XRD analysis the powder from SF-77-P1 was repressed into a pellet and sintered for 5 h at 1450 C. The resulting Nd 1.9 Pu 0.1 Zr 2 O 7 pellet was investigated by scanning electron microscopy (SEM). The pellet was sputtered with Au-Pd to prevent charging of the specimen during examination. A JEOL 6490 LV operating at a working voltage of 30 kV and a working distance of 10 mm was used to image the pellet microstructure. Elemental analysis was performed by energy dispersive spectroscopy (EDX) with a xed Xray acquisition time of 100 seconds.
2.2.2 XAS measurements. XAS data on the Nd 2Àx Pu x Zr 2 O 7 samples (where x ¼ 0.1 and 0.2) and the plutonium nitrate precursor were carried out at the INE-Beamline of the Angströmquelle Karlsruhe (ANKA). 41 Data for the other Pu reference standards were kindly provided by Vitova et al. 42 XANES and EXAFS spectra were recorded at both the Pu L III (18 057 eV) and the Zr K (17 998 eV) edges. The following scanning intervals were used (energy values relative to E 0 ): À170 eV to À30 eV (step size 5 eV, 2 second integration time), À30 eV to 30 eV (step size 0.5 eV, 2 second integration time), and 30 eV to 12.5k (step size 0.035k, 0.5k integration time). Several scans were taken for each sample and summed to improve the signal-to-noise ratio. Energy calibration of the monochromator was performed by  collecting spectra from a Zr metal foil; the edge position of the foil (measured at the rst inection in the derivative of the absorption) was aligned to the expected value of 17 998 eV. Due to the overlap of the Pu and Zr absorption edges data was collected in uorescence mode on sintered pellet samples contained in a double ultra-high-molecular-weight polyethylene (UHMWPE) containment. The INE beamline is a bending magnet beamline which provides an operating X-ray energy range between 2.1 and 25 keV. The optical setup comprises a Ge(422) double crystal monochromator and Rh coated silicon mirrors to focus the beam down to a 500 mm by 500 mm spot.
The incident X-ray intensity was measured using a gas lled ionisation chamber and the emitted uorescence X-ray photons intensity was measured using a 4 element silicon dri detector (Vortex, SII NanoTechnology). For collection of the Zr K a1 uorescence photons (15 775 eV) the detector was windowed over a range of 15 200-16 300 eV. For collection of the Pu L a1 uorescent photons (14 288 eV) the detector was windowed over a narrow range between 14 050 and 14 400 eV. The narrow range was required to remove any interference from the Zr K a photons which were contributing a large signal in this energy region due to the high relative concentration of Zr, in relation to Pu, in the samples. No spectra were recorded at a Nd L III edge (7126 eV) due to the high attenuation of low energy X-rays by the sample and the holder. XAS data on Nd 2 Zr 2 O 7 and the Zr reference standards were collected at the Zr K edge on beamline X23A2 of the now decommissioned National Synchrotron Light Source (NSLS) at the Brookhaven National Laboratory (BNL). The following scanning intervals were used (energy values relative to E 0 ); À200 eV to À20 eV (step size 1 eV, 1 second integration time), À20 eV to 30 eV (step size 0.3 eV, 1 second integration time), and 30 eV to 15k (step size 0.05k, 0.5k integration time). Energy calibration of the monochromator was performed by collecting spectra from a Zr metal foil; the edge position of the foil (measured at the rst inection in the derivative of the absorption) was aligned to the expected value of 17 998 eV. Data were acquired in transmission mode using nely ground powder specimens dispersed in polyethylene glycol (PEG) to give a thickness of one absorption length. X23A2 consisted of an unfocused bending magnet beamline (4.9 to 32 keV) with an optics setup incorporating a Si(311) monochromator with a xed exit Golovchenko-Cowan design, and a single bounce at Rh-coated harmonic rejection mirror. Slits were employed to reduce the beam size to 7 mm in the vertical and 0.5 mm in the horizontal. The incident and transmitted X-ray intensities were measured using gas lled ionisation chambers.
All raw data was processed using the Athena soware and EXAFS analysis was performed using FEFF6 and the Artemis program. 43 The Zr k-edge data was corrected for self-absorption using the Troger algorithm. 44 The Pu L III edge data was not corrected as self-absorption is not a concern when working in the dilute limit. 45 2.2.3 Bond-valence-sum calculations. The bond valence concept provides validation of a crystal structure model by comparison of the formal oxidation state (V i ) with the bond valence sum. According to Brown-Altermatt 46 the bond valence sum is given by: where r ij and r 0 are the observed and tabulated bond lengths, between the atoms i and j and B is an empirical constant (usually 0.37 for oxides). Reference bond lengths were taken from Zachariasen. 47

Molecular simulation
The atomistic modelling studies of Pu incorporation into the Nd 2 Zr 2 O 7 pyrochlore structure were performed using the density functional theory (DFT)-based Quantum-ESPRESSO simulation package. 48 In order to make reliable predictions of the energetics of Pu incorporation the PBE exchange-correlation functional was applied. 49 This methodology has previously been successfully employed in studies of actinide containing systems 50,51 and Nd x Zr 1Àx O 2À0.5x pyrochlores-type compounds, including Nd 2 Zr 2 O 7 . 10,34 As in these previous studies, the PBEsol functional 52 was used to obtain better geometries of selected atomic congurations. The core electrons of the constituent atoms were modelled with ultraso pseudopotentials 53 and the 6s 2 6p 6 6d 2 7s 2 electrons (plus 5f electrons in DFT + U calculations) of the Pu atoms were computed explicitly. Based on previous experience in computing f elements-bearing systems, 50 Ryd was selected as the plane-wave energy cut-off. 51 Pu has strongly correlated 5f electrons therefore the correlation effects were computed using the DFT + U approach as a correction to the regular DFT calculations. This approach has previously been successfully applied to simulate Nd x Zr 1Àx O 2À0.5x pyrochlore-type systems 10 however, because the computed correlation effects for Nd atoms in Nd 2 Zr 2 O 7 are negligibly small, 10 the focus in this study was on the correlation effects associated with the Pu atoms only. The Hubbard U parameter values for Pu(III) and Pu(IV) applied in this study were 2.0 eV and 2.7 eV, respectively. These representative values were taken from Beridze et al. 51 and were derived from a series of Pu-bearing compounds using the linear response method. 54 Following our methodology from previous studies, 10,34,36 the Nd 2 Zr 2 O 7 pyrochlore structure was represented by an 88 atoms cubic supercell and calculations were performed on the 2 Â 2 Â 2 Methfessel-Paxton k-point grids. 55 The solution energies of the Pu atoms was computed assuming the following reactions: Incorporation of Pu(III) on the Nd (A) site: Incorporation of Pu(III) on the Zr (B) site: Incorporation of Pu(IV) on the Zr (B) site: Incorporation of two Pu(IV) on the Nd (A) site with subsequent introduction of an oxygen atom on an oxygen vacancy site: Incorporation of three Pu(IV) on the Nd (A) site with subsequent introduction of one Nd vacancy: Incorporation of Pu(III) on the Nd (A) or Zr (B) site via the reduction of Pu(IV): In all the above reactions the reference reactant state chosen for Pu was PuO 2 . Reactions (2), (3), (7) and (8) would only be relevant under reducing synthesis conditions such as those used during this study. The rationale behind using the same Pu reference state for all reactions was to allow direct comparison of the resulting reaction enthalpies. This approach differs from previous reported modelling studies, [31][32][33] in which Pu 2 O 3 has been used as the reference for reactions involving incorporation of Pu(III). Because different pseudopotentials were applied for Pu(III) and Pu(IV) when performing DFT calculations with 5f electrons included in the pseudopotential core, we used Pu 2 O 3 as the reference for reactions leading to the formation of Pu(III). However, because PuO 2 was used as the reference reactant in all of the above mentioned reactions, the standard enthalpy difference between Pu 2 O 3 and PuO 2 of 232 kJ mol À1 (ref. 56) was added to the relevant reaction enthalpies computed with Pu 2 O 3 . To obtain solution energies for reaction (7) and (8) the standard formation enthalpy of water gas (À241.8 kJ mol À1 ¼ À1.25 eV) was added to the solution energies of reactions (2) and (3), respectively. In order to nd the most energetically favourable Pu incorporation mechanism, allowed by the 88 atoms supercell, Pu incorporation congurations were calculated.

Synthesis routes and X-ray diffraction (XRD)
Synthesis route 1 involved precipitation of the plutonium hydroxide prior to Nd-and Zr-hydroxide formation; this allows the two steps to be studied separately. However, to allow for a co-precipitation of all three hydroxides and the formation of a homogeneous precursor at the molecular level, the metal salt solutions of plutonium, neodymium and zirconium were mixed prior to precipitation, in synthesis route 2. The Nd-and Zr-metal salt solution was acidied and the concentration of the Pusolution was reduced to c(Pu) ¼ 0.01 mol L À1 . All thermal treatments were carried out under reducing conditions with the intention to reduce Pu(IV) to Pu(III), which should occupy the Asite in pyrochlore.
The presence of a Pu(IV) species was reported by Toth et al. 57 for 0.05 mol L À1 Pu(NO 3 ) 4 in 0.11 M HNO 3 . Disproportionation reactions led to the presence of additional Pu species so the Pu(NO 3 ) 4 was always freshly dissolved in 1 M HNO 3 prior to being used. Calculations of the thermodynamic equilibrium Pu species were carried out with GEMS using the NAGRA-PSI database, 58 according to the experimental conditions of synthesis route 1 in the presence of 1 kg water. The calculations indicated aqueous plutonyl PuO 2 2+ to be the predominant species. It should be noted however, that the GEMS calculations can only predict the thermodynamic equilibrium and no kinetic effects of the reaction are taken into account. Clark et al. 59 reported Pu(IV) to be unstable if it comes into contact with oxygen but the rate for this reaction was found to be negligible. The presence of Pu(VI) has also been described by Glazyrin et al. 60 in 1-5 M HNO 3 at 90 C. In the present study the presence of Pu(VI) was not expected as the solutions were always freshly prepared prior to each synthesis and directly consumed. The disproportionation reaction of Pu(IV) in acidic solutions is given below: The fourth order dependence of the disproportionation on [H + ] makes this reaction highly unlikely to occur. Under strongly acidic conditions 59 there was no experimental observation for the disproportionation reaction having taken place. Therefore, the dissolution of Pu(NO 3 ) 4 in 1 M HNO 3 at room temperature is assumed to result in a Pu(IV) solution. In addition, sintering of the resulting powders takes place under reducing conditions which makes the oxidation state in the solution less critical for the nal product.
For both synthesis routes all thermal treatments were carried out under reducing atmosphere to promote the reduction of Pu(IV) to Pu(III) and inhibit the formation of PuO 2 as a discrete phase. For synthesis route 2 the sintering duration was extended to 80 h, because the kinetics of the reduction of Pu(IV) to Pu(III) and the incorporation mechanism of Pu into the pyrochlore structure are not well understood. Two Nd 2 (Zr 1.9 -Pu 0.1 )O 7 pellets were pressed and sintered for 80 h to reduce the plutonium to Pu(III). One of the pellets was ground and the powder characterised by XRD. The XRD patterns obtained for the pure Nd 2 Zr 2 O 7 compound and 5 and 10 mol% Pu doped compounds contained reections which could all be attributed to a pyrochlore structured phase. Fig. 1 shows the pattern acquired from Nd 1.9 Pu 0.1 Zr 2 O 7 which is representative of all the samples. The powder was subsequently pressed and resintering to be used for SEM characterisation.
Previous studies in the literature concerning the fabrication of plutonium pyrochlores lack a thorough analysis of both the structural incorporation and the oxidation state of Pu. Kulkarni et al. 61 prepared a Pu 2 Zr 2 O 7 pyrochlore via a solid state route under helium atmosphere. They ascribed an observed weight loss between 1000-1400 C to the reduction of Pu(IV) to Pu(III) driven by oxidation of graphite present in the system. They observed a linear decrease of the lattice parameter in the series (Pu x La 1Àx ) 2 Zr 2 O 7 (where x ¼ 0.0-1.0) with an increase in Pu content. This is consistent with a reduction in the average size of the A-site cation, because both Pu(IV) and Pu(III) have a smaller ionic radius in eightfold coordination than La(III). Although the authors stated that a weight gain could be observed when the samples were re-oxidised no direct measurement of the Pu oxidation state was provided. Burakov et al. 7 reported the fabrication of Pu-238 doped titanate pyrochlores via a modied solid state route. A plutonium nitrate solution was added to a mixture of inactive oxide precursors and subsequently calcined and sintered in air. No specic details were provided in the publication but it is assumed that denitration of the plutonium nitrate occurred yielding PuO 2, prior to a further solid state reaction resulting in the formation of the target phase. There was no characterisation of the plutonium oxidation state or structural environment and based on the analysis provided it is unclear as to whether the samples were homogenous.

Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX)
The Nd 1.9 Pu 0.1 Zr 2 O 7 pellet prepared via synthesis route 2, was further characterised by SEM and EDX. Fig. 2(a) shows a secondary electron (SE) image of the pellet surface; the areas where EDX spot measurements were made are marked. The elemental distribution was probed by mapping the surface of the pellet. The maps are shown in Fig. 2(c-e) and they indicate a homogeneous distribution of Pu, Zr, and Nd. The surface imperfection, visible on the right side of the pellet in Fig. 2(a), is either an artefact of the sintering process or material deposited during the surface coating process. It was not possible to prepare the surface of the sample using metallographic techniques prior to the analysis but there was no indication that this region was chemically dissimilar to the rest of the pellet.
EDX measurements of the composition, at the four spots of interest in Fig. 2(a) indicated a mean Pu content of 5.4 at%, and that the Pu was distributed homogeneously at locations 1, 2 and 3 within the limit of sensitivity for the technique. The measurement at location 4 was taken from an agglomeration of ceramic grains, with a diameter of about 40 mm, located in the cavity shown Fig. 3(d). The plutonium content at this location was 7.8 at% which is approximately one and a half times higher than the value measured at locations 1, 2, and 3. The microstructure of the four locations were subsequently characterised in more detail. Fig. 3(a-c) show location 1 at three different magnications. The microstructure is composed of primary grains between 1-5 mm in diameter as seen in Fig. 3(c). The pellet surface appears porous and is covered with small grains. This may be caused by the grinding of the pellet and the relatively short sintering time of 5 h aer the repressing step. The difference in the brightness of the smaller grains, with respect to the larger ones, is related to non-uniformities in the conductive surface coating.
There is no indication of any gross differences in chemical composition, or the presence of any secondary phases in the sample, which is consistent with the XRD analysis and conrms that a single phase Pu-pyrochlore has been formed.
The wet-chemical synthesis routes, used in this work, could plausibly result in three possible scenarios for the charge and position of plutonium within the pyrochlore crystal lattice: (1) The synthesis conditions will promote reduction of Pu(IV) in the starting precursor and formation of a pyrochlore with Pu(III) on the A-site. Nästren et al. 17 reported the formation of a single phase Pu-doped neodymium pyrochlore, containing Pu(III), under reduction conditions (Ar/H 2 ). It was not indicated whether the Pu was added in the tri-or tetra-valent state so it is unclear as to whether the processing environment had driven the reduction from Pu(IV) to Pu(III) or simply stabilised the Pu(III) already present. This processing route led to Pu(III) being accommodated in eightfold coordination at the A-site in the pyrochlore structure with no charge compensation required. Gregg et al. reported the synthesis of La 1.9 Pu 0.1 Zr 2 O 7 via a modied alkoxide route using PuO 2 dissolved in nitric acid and observed the formation of a near single phase pyrochlore with Pu(III) present at the A-site. 18 (2) The kinetics of the pyrochlore crystallisation process may be faster than the reduction of the Pu in the precursor from Pu(IV) to Pu(III). This could lead to the formation of the pyrochlore with Pu(IV) occupying the A-site. It is also plausible that Pu(IV) could be incorporated onto the B-site however this would require substitution of Zr onto the A-site which seems unlikely based on the relative ionic size difference between Nd(III) and Zr(IV) in eightfold coordination.
(3) The synthesis conditions are inadequate to reduce Pu(IV) to Pu(III) during the thermal treatment. In this case Pu(IV) could occupy either the A-site or the B-site. Mixed occupancy of both sites is also possible.
To resolve these questions EXAFS and XANES measurements were carried out at the ANKA-INE beamline. Complimentary atomistic simulation studies were also performed. starting material. In contrast to rst row transition metals the XANES region of actinide L-edge spectra do not contain many features making them relatively easy to interpret. The spectra for tri-and tetra-valent compounds tend to be similar in appearance and consist of a single broad absorption peak with smooth rising and falling edges, and a second broad feature at approx. 45 eV above the white line. Because of the lack of features the principle characteristics typically reported are the energy of the peak on the white line and the absorption edge energy (determined from the inection point on the edge). 62 For all data a pre-edge background subtraction was performed and the edges were normalised allowing the edge and peak positions to be measured. 45 As shown in Fig. 4(a) there is a shi in the energy position of the maximum on the white line with a reduction in the Pu oxidation state. Fig. 4(b) shows the rst derivative of the normalised absorption where the inection point (edge position) on the rising edge of the white line is given by the rst maximum. A clear shi in the position of the maximum in the rst derivative can be seen between the two standard solutions. Values for the peak energy and the absorption edge energy for the samples and standards are given in Table 3.
The absorption edge energy is primarily determined by the charge of the central Pu atom although the exact position, and shape, will be modied by the degree of electron transfer to the surrounding ligands. In the case of the lower valence actinides (Act) there is a monotonic increase in the energy of the absorption edge with valence however a discontinuity arises between Act(IV) and Act(V) which is related to the formation of trans-dioxo species. 63,64 For the case of Pu aquo-complexes (prepared in 1 M HClO 4 ) clear differences in the shape of the near-edge spectra for the dioxo-containing species were reported. 65 Multiple scattering (MS) events along the dioxospecies give rise to a feature at approximately 15 eV above the white line which has also been well documented in U and Np compounds. 66,67 The absence of this feature in our data indicates that no short Pu-O bonds are being formed and excludes the presence of dioxo-species in these samples. The shape of the  edge is sensitive to both the local coordination environment and the electronegativity of the connecting ligands. The features in the Act L 3 XANES are therefore determined, to some extent, by the photoelectron scattering paths and not simply by the occupancy of the 6d electronic orbitals. 62 It is therefore necessary to select appropriate standards and the correct methodology for assigning valence; comparison of data from a range of aquo-, aquochloro-, aquonitrato-, and carbonato-complexes indicated that the shi in the inection point on the absorption edge is far more sensitive to changes to the extended chemical environment than the valence of the central Pu cation. 62 These changes can inuence the interpretation of unknown compounds; asymmetry in the white line will inuence the determination of the edge energy, from the rst inection point on the absorption edge, and care should be exercised to allow unambiguous assignment of the valence state. Soldatov et al. demonstrated this effect in tertiary U oxides. 68 As can be seen in Table 3 there is a wide spread in the edge energy position for the Pu(IV) standards but the peak energies are more tightly clustered. It was therefore decided to determine the oxidation state of the samples from the peak energy. An average peak energy was calculated for the Pu(IV) standards and a linear change in peak position with oxidation state was assumed. The results presented here for the standards are in good agreement with those published previously. 18,65 The tabulated values for the sample oxidation states were then determined by interpolation; the results indicate that for both pyrochlore samples the majority of the Pu is the tetravalent oxidation state, which is unexpected given that the samples were red under reducing conditions. The isostructural Pu 2 Zr 2 O 7 and Am 2 Zr 2 O 7 compounds are known to form when heated under a hydrogen (4% in Ar) atmosphere at 1500 C (10-20 hours dwell time). 69 Shoup et al. 70 18 Similar results were also reported by Naestren at low dopant concentration although they used a signicantly higher sintering temperature (1650 C) and a metal furnace. 17 Although our ndings differ from previously published results it is worth considering the fact that differences in Pu content, processing conditions, and heat treatment parameters will have an effect on the nal Pu oxidation state. It should also be noted that the experimental results are consistent with the results of the molecular modelling (Section 3.4). These suggest that the most energetically favourable mechanism is for the Pu to be incorporated as Pu(IV) on the Nd(III) site with the introduction of oxygen atoms into oxygen vacancy sites. Zirconium k-edge XANES. Spectra from a group of Zr containing crystalline standards are shown in Fig. 5(a). For all data a pre-edge background subtraction was performed and the edges were normalised allowing a value for the edge position to be measured; the edge position is herein dened as the energy half way up the absorption edge (i.e. at a normalised absorption value of 0.5). These compounds were chosen to represent the variety of different characteristic rst shell environments that can be found around Zr in mineral systems. It should be noted that the spectra presented here are consistent with those already published previously. [71][72][73][74] Synthetic zircon, ZrSiO 4 (Acros Organics 99%), is a model for Zr in eight-fold coordination. 75 Zr occurs in a disordered seven-fold coordinated site in baddeleyite (ZrO 2 ) 76 where all seven Zr-O bonds are of different length. The baddeleyite used in this study was synthetic reagent grade monoclinic zirconia (Sigma-Aldrich 99%). Zirconia undergoes high temperature phase transitions to tetragonal and cubic structures which can be stabilised at room temperature using oversized dopants such as Y or Ca. In this study synthetic yttria-stabilised zirconia (Tosoh Corporation 99%) and calcium-stabilised zirconia (synthesised in house from ZrO 2 and CaCO 3 ; both Sigma-Aldrich 99%) were measured. The transformation process involves the distortion of the Zr-O nearest neighbour shell and fully stabilised cubic zirconia solid solutions contain seven-fold coordinated Zr. 77 Zr in six-fold coordination is common in Zr-bearing alkali silicate minerals such as zektzerite (LiNaZrSi 6 O 15 ) 78 and catapleiite (Na 2 ZrSi 3 O 9 -$2H 2 O); 79 the coordination polyhedral in these samples is a nearly regular octahedron. The nal standard measured was synthetic BaZrO 3 (synthesised in house from ZrO 2 and BaCO 3 ; both Sigma-Aldrich 99%) which has the ideal cubic perovskite structure where the Zr atoms are situated in regular octahedra.
Inspection of the K-edge XANES region of the spectra from these materials shows marked differences in the shape and relative intensity of the individual components of the doublet (indicated by a and b in Fig. 5(a)) on the absorption edge. 72 The octahedrally coordinated Zr standard (ZrSiO 4 ) is characterised by a larger intensity component on the low energy side of the doublet whilst for the six-fold coordinated standard (BaZrO 3 ) the component on the high energy side is more intense. These two features appear at approx. 6 and 15 eV above the position of the edge (E 0 ) and can be used as a qualitative diagnostic for the average coordination environment of the Zr. When Zr is situated in a highly centro-symmetric environment then the splitting between the two features is far more pronounced; this can be seen clearly in the data obtained from the BaZrO 3 sample. Lee et al. observed that the splitting (DE) increased with increasing crystal symmetry in Gd 2 Zr 2 O 7 samples prepared by the Pechini method and processed at different temperatures. 80 Samples processed at lower temperatures adopted a more disordered structure and had a smaller DE. Similar trends were shown by Li et al. 77 in their study on different zirconia polytypes, indicating that the k-edge absorption spectra provide a useful tool for studying order/disorder. Changes in the relative intensity of the two features also indicate a change in the local Zr structural environment. In the yttria doped zirconia system a slight increase in the relative intensity of a in conjunction with a small shi in b to lower energy was characteristic of an increase in the average Zr-O coordination number. 81 Data acquired at the Zr k-edge for the pyrochlore samples (Nd 2Àx Pu x Zr 2 O 7 ; where x ¼ 0.0, 0.1, and 0.2) are shown in Fig. 5(b) together with the reference spectra from catapleiite and zircon. All three spectra show a marked similarity to catapleiite indicating that the Zr is six-fold coordinated. This result is consistent with the expected crystal structure for Nd 2 Zr 2 O 7 , 82 and the X-ray diffraction results presented in Section 3.1. Semiquantitative inspection of the spectra shows that the splitting between the two features is reduced for the Pu doped samples suggesting that the local Zr environment is being distorted by the introduction of Pu cations on the A-site in the pyrochlore structure. This is not unexpected given the 13.3% difference in ionic radii, between eight-fold coordinated Nd(III) (I R ¼ 1.109Å) and Pu(IV) (I R ¼ 0.962Å). 83 A small contraction in the average size of the A-site alongside an increase in the local structural distortion would be anticipated when Nd is replaced with smaller Pu cations. Changes in the relative intensity of the a and b features are also observed with increasing Pu content consistent with the introduction of Pu(IV) on the Nd(III) site and subsequent charge balance by the lling of oxygen vacancies associated with the Zr.
Plutonium L III -edge EXAFS. Analysis of the extended X-ray absorption ne structure (EXAFS) can reveal information about the local structural environment surrounding different atoms within crystalline and amorphous materials. Conrmation of both the number of next nearest neighbours (coordination number) and also the scattering path length (approximately equal to twice the bond distance) between an atom and those neighbours can be obtained. In well-ordered systems information can also be obtained about more distant coordination shells allowing a more detailed description of the local structure around a particular set of atoms.
The local structural environment around the Pu atoms was investigated by quantitative analysis of the EXAFS region of the Pu L III -edge data. In order to rene the contact distances, between the central absorbing Pu atoms and the nearest neighbour coordination shells, a structural model was required. The structure of rare earth zirconate pyrochlores are well reported in the literature; in this study the structure of Nd 2 Zr 2 O 7 rened by Harvey et al. 82 was used as the starting model. The Pu was targeted for incorporation on the A-site of the pyrochlore structure which is characterised as having eight nearest neighbours; six longer A-O 48f bonds and two shorter A-O 8b bonds. As a rst approximation, a structural model containing two scattering paths was used to represent the two different A-O bond lengths. The degeneracies of the paths were weighted accordingly and a mean square displacement parameter s 2 (O1) was used to model the thermal vibrations of the nearest neighbour oxygens. The amplitude reduction factor (S 0 2 ) and the shi in the edge energy (DE 0 ) were also allowed to vary. It should be noted at this stage the tted k and R ranges were restrained to only model the signal in the FT k 3 c(k) at approx. 1.8Å (uncorrected for phase shi). This model gave a t to the data which was deemed unacceptable due to the large rened value of DE 0 and negative mean square displacement parameters. This result suggests that the Pu rst shell environment is signicantly distorted from that of the classic pyrochlore A-site structural motif. The degeneracies of the two Pu-O1 paths were then inverted to give six short Pu-O1 s paths and two longer Pu-O1 l paths. This approach yielded a reasonable t with sensible values for the S 0 2 , DE 0 , and the mean square displacement parameters. In order to conrm the robustness of the model the electron reduction factor S 0 2 was xed at the value rened previously, and the degeneracy of the two Pu-O1 paths were allowed to vary. This method returned a t where the degeneracies converging towards six for the short Pu-O1 s path and two for the longer Pu-O1 l path. For completeness, an attempt to t a single oxygen shell around the Pu was made. Fitting a structural environment like Zr in Nd 2 Zr 2 O 7 failed returning an unacceptably large S 0 2 and a negative mean square displacement parameter. Efforts to t a single environment with eight nearest neighbour oxygen atoms were also unsuccessful, indicating that the environment around the Pu atoms is signicantly distorted.
In order to satisfactorily t the spectral features at higher radial distance, additional scattering paths were added to the model. This took into account the Pu-Nd, Pu-Zr1, Pu-O2, Pu-O3, Pu-Zr2, and Pu-O4 scattering paths; where applicable an appropriately weighted scattering path was chosen to model all like atom pairs at similar distances. Fractional occupancy of plutonium was incorporated by inclusion of a Pu-Pu scattering path. The available scattering paths are generated by the FEFF algorithm which uses ab initio theory to calculate multiple scattering paths based on a given structural model. 84 The structural model used is taken from a crystallographic input le oen referred to as a CIF. 85 In order to allow FEFF to generate the additional Pu-Pu scattering path the CIF was modied by changing one of the six second shell neighbours from a Nd atom to a Pu atom. This change alone would result in a fractional occupancy of 0.167 formula units Pu on the A-site. In order to correctly model the fractional occupancy, the contribution (amplitude) of the resulting Pu-Pu scattering path was multiplied by a factor of x/(1/6) and the Pu-Nd scattering paths were multiplied by a factor of (1Àx)/(5/6), where x is the fraction of Pu atoms on the A-site. This produced a model which afforded a good t (R-factor ¼ 2%) to the main features in both the k 3 c(k) waveform and the FT k 3 c(k) spectral envelope out to a k of 11Å À1 and an R of 5.5Å, respectively. The ts to both data sets are shown in Fig. 6 and the rened model parameters are shown in Table 4. The data were modelled with thirteen independent variables: two variables related to the overall EXAFS scattering process (S 0 2 and E 0 ) and eleven variables related to nine independent single scattering path lengths and associated mean squared displacement parameters.  82 was again used as the starting model. The Nd 2 Zr 2 O 7 pyrochlore structure is characterised by a Zr site in 6-fold coordination, with six Zr-O bond lengths at 2.09Å. As a rst approximation a structural model containing one single scattering path, to represent the six Zr-O1 bonds, was used. The degeneracy of this path was weighted to six and a mean square displacement parameter s 2 (O1) was used to model the thermal vibrations of the nearest neighbour oxygens. This yielded a satisfactory t to both the k 3 c(k) waveform and the FT k 3 c(k) spectral envelope for the Nd 2 Zr 2 O 7 sample. At this stage the tted k and R ranges were restrained to only model the signal in the FT k 3 c(k) at approx. 1.6Å (uncorrected for phase shi). In order to satisfactorily t the spectral features at higher radial distance, additional scattering paths were added to the model. This took into account the Zr-Nd, Zr-Zr, and Zr-O2 scattering paths at 3.79Å, 3.80Å, and 4.09 A, respectively. As for the rst oxygen shell, an appropriately weighted scattering path was chosen to model all like atoms pairs at similar distances. This approach gave a model which provided a good t (R-factor ¼ 3.0%) to the main features in both the k 3 c(k) waveform and the FT k 3 c(k) spectral envelope out to a k of 13Å À1 and an R of 4Å, respectively. The ts are shown in Fig. 7 and the rened model parameters are shown in Table 5. The data were modelled with nine independent variables: two variables related to the overall EXAFS scattering process (S 0 2 and E 0 ) and seven variables related to ve independent scattering single scattering path lengths and associated mean squared displacement parameters. The Zr-O1 bond length was determined to be 2.10Å AE 0.02, which is in good agreement (a difference of 1%) with the weighted mean bond length from the crystal structure published by Harvey et al. 82 Independent validation of the rened path lengths can be performed using the bond valence sum approach. 46 The bond valence sum determined from the rened EXAFS model was 3.9 AE 0.1 valence units (v.u.), which differs by less than 5% from the formal Zr valence state of 4 v.u. Comparison of the rened Zr-Nd, Zr-Zr, and Zr-O2 paths dened by EXAFS with the mean contact distances determined by crystallography, showed good agreement between the two techniques. The same approach was used to t the EXAFS data from the Pu doped samples. In order to incorporate fractional occupancy of Pu on the A-site, and therefore a contribution from a Zr-Pu scattering path, the model was modied. In order to allow FEFF to generate the additional Zr-Pu scattering path the CIF was modied by changing one of the six second shell neighbours from a Nd atom to a Pu atom. This change alone would result in a fractional occupancy of 0.167 formula units Pu on the A-site. In order to correctly model the fractional occupancy, the contribution (amplitude) of the resulting Zr-Pu scattering path was multiplied by a factor of x/(1/6) and the Zr-Nd scattering paths were multiplied by a factor of (1Àx)/(5/6), where x is the fraction of Pu atoms on the A-site. The (Nd 2Àx Pu x )Zr 2 O 7 (x ¼ 0.1 and 0.2) data sets were tted using this modied model with the results shown in Fig. 7 and Table 5. The modied model provided acceptable ts to the EXAFS data with the Zr-O1 paths length determined to be 2.09Å AE 0.02 and 2.10Å AE 0.02, for the x ¼ 0.05 and 0.10 samples, respectively. This yielded bond valence sums of 3.9 AE 0.1 v.u. and 4.0 AE 0.1 v.u. which were in good agreement with the crystallography. Although the small increases in the Zr-O1 path length is not signicant in view of the magnitude of the associated errors, an increase would be expected if the extra oxygen atoms associated with charge balancing the alio-valent substitution of Nd(III) for Pu(IV) were associated with the Zr atoms. Attempts to introduce a contribution from this additional oxygen atom did not signicantly improve the quality of the t. This was not unexpected given that the content of Pu dopant is ten times lower than that of the Zr atoms. Any impact of the extra oxygen on the average Zr-O1 distance would be very small and difficult to detect due to the Table 4 Refined structural parameters for the pyrochlore samples from the Pu L III edge EXAFS. The average interatomic distance for a given Pu-i pair is denoted as R (Pu-i), the Debye-Waller factor is denoted as s 2 (i) and the number of scatters in the coordination shell is denoted as N(Pu-i). The fitting range in R was 1.5-5.5Å and 3.0-11.5 in k. A dash denotes there is no error associated with a value Nd low occupancy of the vacancy site. The slight increase in the Zr-Nd and Zr-Zr path lengths are not signicant given the magnitude of the associated errors. The large increase in the Zr-O2 path length, and the associated errors is consistent with an increase in the overall disorder around the zirconium atoms, as indicated by the analysis of the XANES region. The immobilisation of Pu(IV) at the eightfold coordinated A site is in contradiction with Pauling's rule, but follows the behaviour described by Verwey and Heilmann. 86, 87 Verwey and Heilmann found that the cation with the highest charge adopted the structural site with the highest coordination number in spinels. Based on this, Pu(IV) would be expected to occupy the eightfold coordinated A site in a pyrochlore.

Molecular modelling
The calculated solution energies of Pu into the Nd 2 Zr 2 O 7 pyrochlore crystal structure, for the reactions given in eqn (2)-(8), are reported in Table 6. The results determined using the force-eld method approach of Cleave et al. 31 and Perriot et al., 33 are  Table 5 Structural parameters for the pyrochlore samples from the Zr k-edge EXAFS. The average interatomic distance for a given Zr-i pair is denoted as R(Zr-i), the Debye-Waller factor is denoted as s 2 (i) and the number of scatters in the coordination shell is denoted as N(Zr-i). The fitting range in R was 1-4Å and 3.0-11.5 in k. A dash denotes there is no error associated with a value  Table 6 show that the force-eldbased solution energies are slightly smaller than the ab initio results derived in this work, and that the 5f electrons correlation effects are rather small (c.a. 0.1 eV). The results suggest that Pu should be preferentially incorporated on the Nd site as Pu(IV) with an additional oxygen per two Pu atoms occupying an oxygen vacancy (reaction (5)). This prediction of Pu(IV) is in agreement with the analysis of the XAS data. Note that Cleave et al., 31 Perriot et al., 33 and Williford & Weber 32 used Pu 2 O 3 as the reference reactant state for Pu, which resulted in a small Pu(III) incorporation energy (À0.12 eV) when Pu(III) occupied the Nd site. The computed DFT energy is À0.01 eV. The approach presented here takes the oxidation step from Pu 2 O 3 to PuO 2 (where DH soln ¼ 2.4 eV) 56 or the reduction process from Pu(IV) to Pu(III) in the pyrochlore structure, into account.
In order to gain more condence about the rather surprising inverted bond distances of Pu-O from the EXAFS measurements, these bond distances were computationally derived. The computed Pu-O, Pu-Nd, Pu-Zr and Pu-Pu bond distances for the different structural congurations and oxidation states of Pu in pyrochlore, are reported in Table 7.
It is important to notice that the calculated Pu-O bond lengths are characterised by a set of short (c.a. 2.3Å) and long (c.a. 2.5Å) bonds for the A site. Pu(IV) incorporated on the Zr site has six short bonds, all of the same length, while Pu(IV) incorporated on the Nd site has three short and ve long bonds; the conguration consists of two neighbouring Pu atoms and an oxygen atom in an associated vacancy ðO 00 v Þ. A similar distribution is computed for the case of Pu incorporation on the A site with the formation of an Nd vacancy ðV 000 Nd Þ to ensure charge neutrality. Incorporation of Pu(III) on the Nd site results in two short and six long bonds. The EXAFS measurements indicate the incorporation of Pu on the Nd site, but with six short (Pu-O1 s ) bonds and two longer (Pu-O1 l ) bonds. The calculated bond distances for the scenarios where Pu(IV) occupies the A site are very close to those obtained from experiment. From the calculated solution energies (Table 6) it is energetically highly unfavourable to incorporate Pu(IV) at the A site with charge balance via Nd vacancy formation. In fact, the calculated solution energies indicate that Pu(IV) will favourably occupy the A site with charge balance occurring via the introduction of oxygen into a former oxygen vacancy. The unexpected results from the EXAFS measurements could indicate a mixture of Pu(IV) on the Nd and Zr sites with approximately equal proportions, which would statistically result in c.a 69% of the bonds being of the shorter length (c.a. 2.3Å). This scenario was investigated by modifying the existing EXAFS tting model to incorporate 50% of the Pu(IV) atoms on each available cation site. Unfortunately, it was not possible to obtain a reliable t as the model now required more independently renable variables than could be supported by the information content of the data (Nyquist theorem). This does not denitely preclude this model, however various parameters had to be xed and the resulting t yielded large S 0 2 and E 0 values in conjunction with negative mean square displacement parameters.

Conclusions
In this paper the fabrication of Pu doped neodymium zirconate pyrochlores via a newly developed co-precipitation route is reported. The co-precipitation approach allowed a homogeneous precursor to be formed which facilitates the uniform distribution of Pu at the molecular level. This precursor was subsequently utilised to synthesis single phase materials in the Nd 2Àx Pu x Zr 2 O 7 system. Analysis of the XAS data determined the Pu to be incorporated as Pu(IV) on the A-site in the pyrochlore structure. These experimental ndings were supported by atomistic simulations which indicate that the lowest solution energy for formation is achieved, when Pu(IV) is substituted on the A-site in the pyrochlore structure, and oxygen atoms are Table 6 The solution energies of Pu in pyrochlore assuming different substitution mechanisms and nominal oxidation states. The charge balance mechanism is indicated in parentheses using Kröger-Vink notation. 88 Note that we computed the energies taking PuO 2 as a reference reactant state for Pu as discussed in Section 2.3. In order to compare the results of different studies 31 the standard enthalpy difference between Pu 2 O 3 and PuO 2 of 232 kJ mol À1 (2.40 eV) 56 was added to the data of Cleave et al. 31 The reported energies are given in eV. In parentheses we report the effect of correlation taken as a difference between the DFT + U and DFT values introduced at vacancy sites to charge balance. Further analysis showed that the local coordination environment of the Pu(IV) consists of six short and two long Pu-O bonds, which differs from the A site coordination environment in rare earth zirconate pyrochlores. The incorporation of Pu(IV) on the A-site results in signicant distortion of the A-site local geometry; this observation is consistent with the smaller ionic radius of Pu(IV), in comparison to Nd(III), and the difference in ionic charge and corresponding crystal eld energies. BVS calculations corroborated the calculated bond distances and oxidation state assignment. Analysis by XRD conrmed that the long range structure of the samples was consistent with the pyrochlore structure and conrmed the uptake of Pu(IV) into the structure. The use of precursors generated by novel coprecipitation routes, such as the one reported here, offers the potential for fabrication of samples with an improved degree of chemical homogeneity on a smaller length scales than those produced by more conventional solid state routes.

Conflicts of interest
There are no conicts to declare.