Article Cite This: Inorg. Chem. 2019, 58, 9118−9126
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Plutonium and Americium Aluminate Perovskites Jean-François Vigier,*,† Karin Popa,*,† Laura Martel, Dario Manara, Oliver Dieste Blanco, Daniel Freis, and Rudy J. M. Konings
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Joint Research Centre (JRC), European Commission, P.O. Box 2340, 76125 Karlsruhe, Germany
ABSTRACT: Both AmAlO3 and PuAlO3 perovskites have been synthesized and characterized using powder X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). AmAlO3 perovskite showed a rhombohedral configuration (space group R3̅c) in agreement with previous studies. The effect of americium α-decay on this material has been followed by XRD and 27Al MAS NMR analyses. In a first step, a progressive increase in the level of disorder in the crystalline phase was detected, associated with a significant crystallographic swelling of the material. In a second step, the crystalline AmAlO3 perovskite was progressively converted into amorphous AmAlO3, with a total amorphization occurring after 8 months and 2 × 1018 α-decays/g. For the first time, PuAlO3 perovskite was synthesized with an orthorhombic configuration (space group Imma), showing an interesting parallel to CeAlO3 and PrAlO3 lanthanide analogues. High-temperature XRD was performed and showed a Imma → R3̅c phase transition occurring between 473 and 573 K. The thermal behavior of R3̅c PuAlO3 was followed from 573 to 1273 K, and extrapolation of the data suggests that cubic plutonium perovskite should become stable at around 1850 K (R3̅c → Pm3̅m transition).
1. INTRODUCTION
orthorhombic Pbnm (Figure 1). Transitions among these three phases depend on the radius of the lanthanide cation and the temperature. However, CeAlO3 and PrAlO3 show peculiar behavior, with further “low-temperature” perovskite distortions starting with the Imma orthorhombic phase (from 430 K for CeAlO316,17 and 205 K for PrAlO318,19). At lower temperatures, the CeAlO3 and PrAlO3 structures exhibit further tetragonal or monoclinic perovskite distortions, including the room-temperature phase of CeAlO3, which are not described here. The formation of these low-temperature phases is surprising, because the neighboring compounds LaAlO3 and NdAlO3 keep rhombohedral configuration R3̅c at least down to 4.220 and 1 K,21 respectively. Therefore, their formation seems to be driven by phenomena other than purely steric effects induced by the ionic radius. This is also confirmed by the absence of low-temperature phases in the case of (Nd1−xLax)AlO3 solid solutions.15 AmAlO3 was previously reported to have a rhombohedral structure, but no structural refinement was performed.23 The local structure of americium in AmAlO3 was investigated by Walter et al.24 using EXAFS, on both crystalline and
Space missions destined to cold, dark, and distant places in the solar system cannot rely on the sun to supply energy. Therefore, numerous space missions have exploited the decay energy of plutonium-238 for heat or electricity production using radioisotope heater units (RHUs) and radioisotope thermoelectric generators (RTGs).1−3 However, 238 Pu is very expensive to produce and suffers from shortages.4 For this reason, the European Space Agency (ESA) is exploring the possibility of using americium-241 instead of 238Pu for future space missions.5−7 To be introduced in a RHU or RTG, americium has to be present in the form of a stable ceramic.8 For this purpose, the use of pure americium oxide does not seem appropriate due to phase instabilities.9−13 Therefore, alternative americium compounds are being investigated,14 for example, americium aluminate. In contrast to actinide aluminates, lanthanide aluminates have been extensively studied in the past, and data were reviewed by Vasylechko et al.15 They give a good overview of what can be expected for actinide aluminates because lanthanides and actinides show generally similar chemical behavior. LnAlO3 compounds mostly crystallize in three main perovskite structures, cubic Pm3̅m, rhombohedral R3̅c, and © 2019 American Chemical Society
Received: March 8, 2019 Published: June 27, 2019 9118
DOI: 10.1021/acs.inorgchem.9b00679 Inorg. Chem. 2019, 58, 9118−9126
Article
Inorganic Chemistry
200 °C/h) under an Ar/H2 atmosphere to 1500 °C with a dwell time of 6 h in an alumina crucible. Because ∼10% of the PuO2 fraction has been detected by X-ray diffraction (XRD) in this specimen, it was crushed again, mixed with a new amount of Al2O3, and thermally treated under identical conditions. The final product consisted of pure PuAlO3 according to X-ray diffraction analysis and was dark violet in color. To use LaAlO3 and NdAlO3 as a reference sample for Raman spectroscopy and LaAlO3 as a diamagnetic reference sample for 27Al MAS NMR, these two compounds were synthesized by solid state reaction. Al2O3 (Aldrich, 99.99%) was mixed with stoichiometric amounts of La2O3 (Alpha Aesar, 99.99%) and Nd2O3 (Alpha Aesar, 99%) and crushed in a mortar. The mixtures were heated in a furnace under air at 1273 K for 12 h, then at 1673 K for 5 h, and finally at 1873 K for 1 h, with intermediate grinding steps between each heat treatment. The phase purity was confirmed by powder XRD analyses, with LaAlO3 and NdAlO3 crystallizing in rhombohedral R3̅ c perovskite structures in agreement with literature data.15 2.2. Characterization Techniques. 2.2.1. X-ray Diffraction. XRD analyses were performed on ∼10 mg of powder loaded in an epoxy resin to avoid any dispersion of radioactive material. A Bruker D8 Advance diffractometer (Cu Kα radiation, 40 kV, and 40 mA) with a Bragg−Brentano θ/2θ configuration was used for the analyses. The device was installed in a glovebox designed for handling of radioactive materials. It was equipped with a curved Ge monochromator (111) and a Lynxeye linear position sensitive detector. The powder patterns were recorded using a step size of 0.013° across the 2θ angle range of 20−120°. High-temperature XRD spectra of PuAlO3 were recorded using a second Bruker D8 X-ray diffractometer mounted with a curved Ge (1, 1, 1) monochromator, a copper ceramic X-ray tube (40 kV, 40 mA), and a LinxEye position sensitive detector and equipped with an Anton Paar HTK 2000 chamber. Approximately 10 mg of powdered PuAlO3 was dispersed on the platinum sample holder using a few drops of isopropanol. Measurements were conducted up to 1373 K under argon, in the 2θ angle range of 20−120° with a 2θ step size of 0.021°. Structural analyses were performed by the Rietveld method using Jana2006.27 Peak profile fitting was achieved using pseudo-Voigt functions. 2.2.2. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR spectra were measured using a Bruker Alpha spectrometer employing the attenuated total reflectance (ATR) technique with a resolution of approximately 4 cm−1 in a range of 400−4000 cm−1. 2.2.3. Raman Spectroscopy. Raman spectra were measured with a Jobin-Yvon T64000 spectrometer used in the single-spectrograph configuration. The excitation source was a Kr+ Coherent continuous wave laser radiating at 647 nm, with a controllable nominal power, usually set to 114 mW at the exit of the cavity, corresponding to approximately 10 mW at the sample surface. The highly radioactive samples were confined in an α-tight capsule closed by a 1 cm thick quartz window, through which the Raman signal was collected.28,29 Spectra were measured in a confocal microscope with a 50-fold magnification and a long focal distance (1 cm). This feature permits a good spectral resolution (±2 cm−1) independent of the surface shape, with a maximum spatial resolution of 2 μm × 2 μm on the sample surface. The spectrograph angle was calibrated with the T2g excitation of a silicon single crystal, set at 520.5 cm−1.30 The instrument is calibrated on a daily basis prior to measurement. Typical integration times ranged from 30 to 60 s with three cycles. 2.2.4. Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). All of the 27Al MAS NMR experiments were performed on a 9.4 T Bruker spectrometer at the Larmor frequency of 100 MHz using a dedicated NMR probe embedded in a nuclearized glovebox.31 The spectra were recorded using a short single pulse with a π/12 length as required for such quadrupolar nuclei.32 All spectra were referenced to 0 ppm using a 1 M AlCl3(l) external reference. The spectra were all fitted using the dmfit software.32 2.2.5. Transmission Electron Microscopy (TEM). Transmission electron microscopy analyses were performed on a FEI Tecnai G2 TEM instrument equipped with a BM-Ultrascan Camera, a high-angle
Figure 1. Schematic presentation of the phase relations in the LnAlO3 perovskite compounds (Ln = La−Gd) based on reported experimental data.15 Ionic radii were taken from the Shannon table.22 Ionic radii of Pu3+ and Am3+ cations are also reported for comparison. The area marked with a star contains further tetragonal and monoclinic perovskite distortions that are not described here.
amorphous compounds. PuAlO3 was also reported to have a rhombohedral lattice by Russel.25 However, the pure phase was obtained from only condensed material recovered from the cold part of the tubular furnace. More standard synthesis methods using solid−solid reaction gave extra unindexed diffraction peaks. Furthermore, as mentioned by Keller et al.,23 mistakes were probably made in the reported lattice parameter values because they show an extreme deviation from other isostructural aluminate compounds and because the parameters given in the hexagonal and rhombohedral cells are not consistent with each other. More recently, Fullarton et al.26 predicted PuAlO3 perovskite to have an orthorhombic Pbnm structure based on density functional theory (DFT) and empirical potential calculations. In this study, we are giving a detailed characterization of AmAlO3 as a potential americium-bearing material for space power sources. Structural and spectroscopic descriptions are provided, as well as behavior of the material under oxidizing conditions and under self-irradiation. The study was extended to PuAlO3, because this material is relatively poorly described in the literature.
2. EXPERIMENTAL SECTION Caution! Americium and plutonium present considerable radiotoxicity hazards. They were handled under carefully controlled dedicated laboratories at the Joint Research Center in Karlsruhe. All steps were performed in hermetically sealed gloveboxes maintained under a slight pressure to ensure confinement of radioactive material at all times. 2.1. Synthesis. Polycrystalline AmAlO3 and PuAlO3 were obtained by conventional solid state reactions using procedures derived from refs 23 and 25 respectively. For AmAlO3, a stoichiometric mixture of 241AmO2 (containing ∼7% 237Np and ∼2% 239Pu) and Al2O3 (Aldrich, 99.99%) was ground in an agate mortar and pressed into a disk. This disk was fired (heating and cooling rates of 200 °C/h) under an Ar/H2 atmosphere to 1400 °C with a dwell time of 6 h in a molybdenum crucible. The final product consisted of brown AmAlO3 and ∼9% fluorite-like dioxide that will be further described in the Results and Discussion. For PuAlO3, a stoichiometric mixture of 239PuO2 (∼99.7% pure) and Al2O3 (Aldrich, 99.99%) was ground in an agate mortar and pressed into a disk. This disk was fired (heating and cooling rates of 9119
DOI: 10.1021/acs.inorgchem.9b00679 Inorg. Chem. 2019, 58, 9118−9126
Article
Inorganic Chemistry
Figure 2. TEM analysis of an americium sample showing EDS spectra for two different particles. The first, marked in red, contains no neptunium and presents an aluminum:americium ratio of ≈1. The second particle, marked in blue and containing no aluminum, presents a neptunium:americium ratio of ≈1. Copper peaks appear due to the copper grid used for TEM analyses and is not related to the sample. annular dark field detector (HAADF), and an energy dispersive X-ray spectroscopy detector (EDS). The field emission gun was operated at 120 kV; the microscope has been adapted for the study of radioactive materials by inserting during its fabrication a flange in the octagon that hosts the objective lenses and attaching a glovebox to it.33
Table 1. Structural Refinement Parameters for AmAlO3 (R3̅c) and PuAlO3 (Imma and R3̅c)a temperature (K) space group space group number a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) M (g/mol) Z crystallographic density goodness of fit (GOF) Rp (%) Rwp (%)
3. RESULTS AND DISCUSSION 3.1. Characterization of AmAlO3 and PuAlO3. XRD analysis showed ∼9% oxide is present with the americium perovskite after the synthesis. To assess the composition of these two phases, TEM was performed on this sample (Figure 2). The analyses show a clear segregation of the 237Np impurity initially present in the americium (daughter isotope of 241Am) within the oxide phase with a ratio between neptunium and americium (Np:Am) of ≈53:47 (a variety of similar ratios have been observed on several particles). Considering this measurement and the oxide proportion determined through XRD, it corresponds to an overall neptunium composition of ∼6% in the sample, which is in good agreement with the level of 7% expected from the of initial americium starting material used for the synthesis. No neptunium was detected in the AmAlO3. This segregation phenomenon is explained by the extreme stability of the Np(IV) oxidation state in oxide-based materials, which is not reduced to oxidation state III, and the nonintegration of tetravalent elements in the aluminate-based perovskite compounds. Finally, particles containing only aluminum were also observed during TEM analyses, corresponding most likely to unreacted Al2O3. This phase was not detected by XRD, probably due to its low concentration and the small contribution of this light material with this technique. The Rietveld refinement results for AmAlO3 and PuAlO3 at room temperature are listed in Tables 1 and 2. AmAlO3 crystallizes in the rhombohedral R3̅c space group at room temperature, in good agreement with previously reported results of Keller23 or Walter.24 PuAlO3 adopts an orthorhombic Imma configuration at room temperature. It is worth noting that the obtained perovskite structure differs from that previously reported by Russel25 (rhombohedral perovskite R3̅ c) and from the calculations by Fullarton et al.26(orthorhombic Pbnm-GdFeO3 type). However, for the latter
AmAlO3
PuAlO3
PuAlO3
298 R3̅c 167 5.33346(7) 5.33346(7) 12.9260(2) 90 90 120 318.429(8) 316.04 6 9.888 2.06 1.14 1.81
298 Imma 74 5.31390(9) 7.50188(11) 5.35287(8) 90 90 90 213.388(6) 314.03 4 9.775 2.76 3.39 5.05
573 R3̅c 167 5.34875(12) 5.34875(12) 13.0137(4) 90 90 120 322.431(14) 314.03 6 9.704 1.94 7.32 9.9
R p = ∑ i [|y i (obs) − y i (calc)|]/∑ i [y i (obs)] × 100. R wp = (∑i{wi[yi(obs) − yi(calc)]2}/∑i[wi × yi(obs)2])1/2 × 100. GOF = Rp/Rexp, where Rexp = {∑i[wi × yi(obs)2]/(n − p)}1/2 × 100.
a
case, it seems that the possibility of Imma configuration formation was not considered in the DFT calculations. Furthermore, DFT calculations refer to 0 K. The formation of the orthorhombic Imma perovskite for PuAlO3 is very interesting because it shows that plutonium aluminate has a peculiar “low-temperature” structure similar to those of CeAlO3 and PrAlO3 (Figure 1). As seen for cerium and praseodymium analogues, a phase transition from Imma to R3̅c structure is observed with an increase in temperature (Figure 3a). The structural refinement of the PuAlO3 R3̅c structure was performed for 573 K, and the results are listed in Tables 1 and 2. The R3̅c PuAlO3 lattice variation showed an anisotropic thermal dilatation in the range of 573−1273 K. It is known that the transition from rhombohedral to cubic perovskite (R3̅c → Pm3̅m) is continuous and is reached when the c/a ratio in the hexagonal lattice is equal to √6.15 Therefore, the 9120
DOI: 10.1021/acs.inorgchem.9b00679 Inorg. Chem. 2019, 58, 9118−9126
Article
Inorganic Chemistry
Table 2. Atomic Coordinates and Isotropic Displacement Parameters for AmAlO3 (R3̅c) and PuAlO3 (Imma and R3̅c) atom PuAlO3, 298 K, Imma Pu1 Al1 O1 O2 PuAlO3, 573 K, R3̅c Pu1 Al1 O1 AmAlO3, 298 K, R3̅c Am1 Al1 O1
y
z
U (Å2)
0 0 0 1/4
1/4 0 1/4 −0.010(11)
0.4958(8) 0 −0.032(9) 1/4
0.0067(9) 0.007(2) 0.017(10) 0.021(6)
1 1 1
0 0 0.529(10)
0 0 0
1/4 0 1/4
0.009(2) 0.009(5) 0.01
1 1 1
0 0 0.534(6)
0 0 0
1/4 0 1/4
0.0058(14) 0.002(3) 0.003(5)
Wyckoff position
occupancy
4e 4a 4e 8g
1 1 1 1
6a 6b 18e 6a 6b 18e
x
perovskite has been synthesized under reducing conditions at 1773 K, it is clear that the observed decomposition is driven by oxidation of Pu(III) to Pu(IV) and not by instability of the plutonium perovskite itself at high temperatures. The behavior of AmAlO3 and PuAlO3 under oxidizing conditions has been tested with a thermal treatment up to 1773 K in pure oxygen. The final products obtained after these treatments have been identified by XRD analysis (Table 3). Table 3. Behavior of AmAlO3 and PuAlO3 after Oxidation Treatment
AmAlO3 PuAlO3
maximum temperature (K)
atmosphere
XRD identification after treatment
1773 1773
O2 (1 atm) O2 (1 atm)
AmAlO3 PuO2 + Al2O3
Under these conditions, PuAlO3 decomposes into PuO2 and Al2O3 whereas AmAlO3 is not oxidized. The latter result shows that the oxidation state (III) of americium is stabilized in the AmAlO3 perovskite, which is beneficial for its use as an americium-bearing material. In the case of application in power sources for space exploration, this suggests that the material would be stable under the conditions to which it will potentially be exposed (vacuum of space, sintering condition, or accidental conditions with atmospheric exposure). The 27Al MAS NMR spectra of LaAlO3, AmAlO3, and PuAlO3 are presented in Figure 4, and their corresponding NMR parameters are listed in Table 4. All spectra reveal one main peak in agreement with their crystalline structure. An impurity is nonetheless detected in AmAlO 3, with a contribution of ∼15% of the 27Al MAS NMR signal, and corresponds most likely to unreacted Al2O3 reagent, in agreement with TEM observations. Indeed, because ∼10 mol % actinides are still present in the sample as dioxide, one can expect the same proportion of unreacted Al2O3. This amount represents
