X-ray absorption fine structure spectroscopic study of uranium nitrides
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- Poineau, F., Yeamans, C.B., Silva, G.W.C. et al. J Radioanal Nucl Chem (2012) 292: 989. doi:10.1007/s10967-011-1551-7
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Uranium mononitride (UN), sesquinitride (U2N3) and dinitride (UN2) were characterized by extended X-Ray absorption fine structure spectroscopy. Analysis on UN indicate the presence of three uranium shells at distances of 3.46(3), 4.89(5) and 6.01(6) Å and a nitrogen shell at a distance of 2.46(2) Å. For U2N3, two absorbing uranium atoms at different crystallographic positions are present in the structure. One of the uranium atoms is surrounded by nitrogen atoms at 2.28(2) Å and by uranium atoms at 3.66(4) and 3.95(4) Å. The second type of uranium atom is surrounded by nitrogen atoms at 2.33(2) and 2.64(3) Å and by uranium atoms at 3.66(4), 3.95(4) and 5.31(5) Å. Results on UN2 indicate two uranium shells at 3.71(4) and 5.32(5) Å and two nitrogen shells at 2.28(2) and 4.34(4) Å. The lattice parameters of UN, U2N3 and UN2 unit cells were respectively determined to be 4.89(5), 10.62(10) and 5.32(5) Å. Those results are well in agreement with those obtained by X-Ray diffraction analysis.
Nitride fuels are a proposed fuel matrix for a number of advanced reactor designs and are an attractive option for transmutation of plutonium and other minor actinides in advanced reactors. Nitrides are considered as potential fuels because they have high solid phase solubility for early actinides and excellent thermal and mechanical characteristics. Among ceramic nuclear fuel matrices, nitrides have the highest metal density, and melting point. The metal fraction of nitride fuel matrices is only slightly lower than that of carbide fuel matrices. Thermal conductivity is slightly lower than carbides but significantly higher than oxides. Incorporation of actinides in zirconium nitride matrix has already been proposed for preparation of transmutation targets. In this context, the local structure of uranium in ZrN has been characterized by extended X-Ray absorption fine structure spectroscopy (EXAFS) spectroscopy . Uranium dinitride (UVIN2), mononitride (UIIIN) and sesquinitride (U2III/VIN3) are the common nitrides of this system. Uranium mononitride and dinitride are indexed in the cubic 3mFm space group, while U2N3 in the cubic body centered space group 3Ia . Because of their interest in nuclear applications, these compounds have been studied intensively and characterized by many techniques such as X-ray powder diffraction (XRD), neutron powder diffraction, nuclear magnetic resonance, transmission electron microscopy and X-Ray photoelectron spectroscopy [3–6]. EXAFS has proved effective among techniques used for structural characterization in materials. EXAFS has been used to determine the effect of oxygen stoichiometry on structure in uranium oxides and to study the formation of solid solution in (U, Ce) O2, (Th, U) O2, (Th, Pu) O2 [7–9]. Very few XAFS characterization has been done on uranium nitrides. Considering the interest in uranium nitrides for nuclear applications, conducting an EXAFS study is important in developing a more complete body of knowledge on these materials. In the present work, the local structure of uranium in uranium mononitride (UN), sesquinitride (U2N3) and dinitride (UN2) was investigated by XAFS spectroscopy and the structural parameters compared with those obtained by diffraction techniques.
Uranium dioxide was purchased from International Bio-Analytical Industries, Inc. (Boca Raton, Florida) and was used as received. Ammonium bifluoride (reagent grade) was purchased from Fischer Scientific and used as received. Ammonia (experimental grade) and argon gas (ultra-high-purity, 99.9999%) used for synthesis were purchased from Praxair.
UN2. Uranium dioxide (10.4 g) and ammonium bifluoride (9.2 g) were ground in a mechanical ball mill for 20 min. The resulting product was bottled and left undisturbed for 55 days, at which point the product was confirmed by XRD as (NH4)4UF8. A charge of 200 mg (NH4)4UF8 was loaded in a platinum boat and placed inside a quartz tube. Quartz fittings sealed the tube and allowed gas to be flowed over the sample. After flushing with argon for 10 min, the cover gas was switched to ammonia. The tube furnace reached 800 °C in 30 min, held at temperature for 1 h, and then cooled to room temperature. The powder XRD confirmed the formation of single-phased UN2. XRD shows the cubic UN2 lattice parameter to be 5.3050(2) Å, matching that of pure UN2 and confirming phase purity and corresponding to an exact composition of UN1.83 .
U2N3 (218.7 mg) was synthesized by heating UN2 (225.4 mg) under an argon atmosphere at 700 °C for 60 min. Purity of the sample was checked by XRD. Initial XRD studies showed this sample to contain U2N3 with a lattice parameter of 10.6691(1) Å.
UN. The mononitride was prepared by heating uranium dinitride synthesized as previously described. UN2 (35.8 mg) was heated at 1,100 °C for 30 min under an argon atmosphere. The resulting product was characterized by XRD and indicated the sample to contain UN (95 wt%) and UO2 (5 wt%). The UN cubic lattice parameter was determined to 4.8900(2) Å.
EXAFS measurements were performed at the advanced photon source (APS) at the BESSRC-CAT 12 BM station at Argonne National Laboratory. Samples were prepared by dilution of compounds in boron nitride (0.5–1% mass). XAFS spectra were recorded at the uranium LIII edge (17 166 eV), in fluorescence mode at room temperature using a 13 elements germanium detector. A double crystal of Si [1 1 1] was used as a monochromator. Energy calibration was done using a Zr foil (K edge = 17 998 eV). For each sample, several EXAFS spectra were recorded in the k range [0–14] Å−1 and averaged. The background contribution was removed using Athena  software and data analysis performed using WINXAS . For the fitting procedure, amplitude and phase shift function were calculated by FEFF8.2 . The Input files were generated by ATOMS  using crystallographic structures of uranium nitrides taken from literature . The adjustments of EXAFS spectra were performed under the constraints S02 = 0.9, a single value of energy shift ΔE0 was used for all scattering. The uncertainty on the coordination number (C.N) is 20%; the uncertainty on the distance (R) is 1%.
Results and discussion
Structural parameters obtained by adjustment of the k3-EXAFS spectra of UN
Inter-atomic distances (Å) determined by EXAFS and XRD in UN
A second EXAFS analysis was performed on a nitride sample containing 15 wt% of UO2 and 85 wt% of UN. Fitted EXAFS parameters are presented in Supplementary Data (Table S2). In this sample, the lattice parameters determined from EXAFS is 4.88(5) Å. The presence of UO2 in the system does not influence the unit cell of UN enough to be detected by EXAFS analysis. It indicates that UO2 is not incorporated in UN up to the observed inter-atomic distances, but is present as a separate crystalline phase. Microscopic characterizations that showed the formation of UO2 as a secondary chemical phase on UN particle surfaces also confirmed this observation .
The number of uranium atoms around U0A was fixed to 75% of the theoretical value.
The number of atoms around U0B was fixed to 25% of the theoretical value.
The distances U0A–U1, U0A–UB1, U0B–U4 and the corresponding σ2 were correlated to a same value.
The distances U0A–U2, U0A–UB2, U0B–U5 and the corresponding σ2 were correlated to a same value. All the other parameters were allowed to vary.
Structural parameter obtained by adjustment of the k3-EXAFS spectra of U2N3
Inter-atomic distances (Å) determined by EXAFS and XRD in U2N3
Structural parameter obtained by adjustment of the k3-EXAFS spectra of UN2
Interatomic distances (Å) determined by EXAFS and XRD in UN2
Uranium nitrides (UN, U2N3, UN2) were characterized by X-ray absorption fine structure spectroscopy. The very good agreement of the experimental structural parameters with reference values indicates that XAFS spectroscopy is a suitable technique for the analysis of uranium nitrides. This is particularly important for future studies of localized bonding environments of mixed nitride fuels of uranium containing fission products and higher actinides incorporated into the UNx matrix. Changes to the local bonding environments within the fuel matrix can be quantify by measuring the EXAFS spectra of Uranium in a matrix containing fission products/actinides in various quantity. Extent of the local matrix distortion can affect the thermal and mechanical properties of the fuel. Study of the EXAFS of un-irradiated materials will provide valuable means to quantify these effects under a wide variety of fuel burnup percentages. In the context of development potential nuclear fuels, plutonium and neptunium nitrides are also considered. Recently, neptunium nitrides were successfully synthesized in our laboratory and characterization of those compounds by XAFS spectroscopy will be subject of further investigation.
Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.