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EXPERIMENTAL
Samples: Thin Films Production. Thin films of epitaxial UO2 were produced by reactive sputtering onto LSAT and YSZ substrates with three different crystallographic orientations: (001), (110) and (111).71

A dedicated DC magnetron sputtering facility with UHV base pressure (10-9 mbar) was employed to grow the films. A depleted uranium metal target was used as a source of uranium. It was kept at a power of 50 W by controlled direct current of 0.11-0.14 A and the corresponding voltage of 350- 450 V, giving a growth rate of 0.9-1.1 Å/s for films on the LSAT substrates, and by controlled direct current at an average value of 0.15 A and the corresponding voltage of 330 V, giving a growth rate of about 1.5 Å/s for films on the YSZ substrates. Argon was used as the sputtering gas at a pAr in the range of 7 to 8 × 10−3 mbar. Oxygen was used as the reactive gas at a pO2 in the range 3.4 to 4.4 × 10−5 mbar for films on the LSAT substrates and at a pO2 of 2 × 10−5 mbar for films on the YSZ substrates, except for sample OB6 for which a pO2 was 3 × 10−6 mbar. The LSAT and YSZ substrates were kept at a temperature close to 750 °C and to 600 °C, respectively.



The substrates were one side polished single crystal LSAT or YSZ with dimensions of 10×10×0.5 mm supplied by MTI Corp, USA. LSAT has a cubic perovskite structure with =3.868 Å,72 and UO2 has a cubic fluorite structure with =5.469 Å, both at room temperature.71 This results in the epitaxial relationship in which the (001) plane of UO2 is rotated by 45° in relation to the (001) plane of LSAT so that the (110) plane of UO2, with a d-spacing =3.867 Å, fits the LSAT (001) plane, with a d-spacing of 3.868 Å (=), as was described by Bao et al.71 for a UO2 film on a LaAlO3 substrate. This causes the UO2 lattice to be only at a slight tension of +0.03% with respect to the substrate in-plane spacing. This 45° rotation epitaxial relationship only holds between the LSAT and UO2 (001) planes. YSZ has a cubic fluorite structure with =5.139 Å.73 This results in the plane to plane epitaxial match in which the plane of UO2 is put at compression of -6.4% by the plane of YSZ. This plane to plane epitaxial relationship holds for the (001), (110) and (111) plane orientations. Table 1 summarizes the produced samples. Sample pairs AP1/OB1 to AP4/OB4 were produced by cutting one sample into two halves using a diamond saw for various studies.

It was identified by means of EDX and XPS analyses that the films of uranium dioxide contain Nb. Based on the results from EDX, XRD, EBSD, SEM and XPS measurements, it is suggested that Nb is present in the form of Nb2O5 and is located in particulates, which precipitated onto the substrates during growth of the films. The particulates can be seen in SEM images (not shown) obtained at different angles. They have sizes down to 30 nm and are densely populated. Niobium concentration was determined by EDX technique, with the point-analysis spot size of 1-2 µm in diameter, at different locations on the surface of the films (~3.5 wt%) which agrees with the concentration values obtained from XPS (~5 wt%). Since the analysis spot in XPS is an ellipse with the minor and major axes of 300 µm and 700 µm, respectively, it was not possible to find an area free of niobium oxide. That is why Nb2O5 lines were observed in XPS spectra from the studied samples which had a relatively small width Γ(Nb 3d5/2)=1.2 eV and Eb(Nb 3d5/2)=206.9 eV. This agrees with the fact that the oxidation state of Nb in the samples is only Nb5+. The work by Fu et al.19 also shows that regions of UO2 and Nb2O5 are formed during the oxidation of uranium-niobium alloy with oxygen. Niobium oxide is not observed in XRD scans of the samples, possibly, due to its low concentration in the films. This observation is consistent with the results obtained in the work by Strehle et al.,73 where the co-deposition of U and Nd was performed onto YSZ substrates. The XRD scans did not exhibit any difference between pure UO2 and UxNdyOz samples, which can be related to the absence of the crystal structure involving neodymium.



Crystallographic orientations for the UO2 films were determined by means of XRD (θ-2θ scans with 𝝋 rotation) and EBSD techniques (not shown). In addition, UO2 films on the (001) YSZ substrates, produced under similar conditions, were thoroughly characterized by Strehle et al.73 and it was shown that these films are single crystals. Since the epitaxial relationship and lattice mismatch for the UO2 films on the (111) and (110) YSZ substrates are the same as for the UO2 films on the (001) YSZ substrate and based on the obtained XRD and EBSD results, it is possible to suggest that these films are also single crystals. Based on the expected epitaxial relationship for UO2 films on the (001) LSAT substrates and the obtained XRD and EBSD results, we are inclined to suggest that the films on the (001) LSAT substrates can be considered as single crystals. As an example, Figure 1 illustrates a result of the EBSD study for sample AP3, which confirms formation mainly of a single crystal with the surface orientation (001). Sample AP2 provides similar results. Uranium dioxide films on the (111) and (110) LSAT substrates are described as preferentially oriented.




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