Determination of interstitial oxygen atom position in U2N3+xOy by near edge structure study

Chemical State of U in U–N–O Ternary System from First-Principles Calculations

“…Since the ratio of U 5+ and U 4+ in the AFM UN 1.5 is 1:1, the reduced formula can be expressed as U 0.5 5+ U 0.5 4+ N 1.5 3– . It has been verified that the structural motif of α-UN 1.5 is [U-U 6 ]­N 6 (the central U atom and six N atoms solely belong to this cluster, whereas the other six U atoms are shared by two clusters, which can be reduced to U 4 N 6 , i.e., U 2 N 3 ), as confirmed by EXAFS . Therefore, both the FM and AFM type UN 1.5 can be further expressed as [U 5+ -U 2 5+ U 4 4+ ]­N 6 3– .…”

Phase and defect evolution in uranium-nitrogen-oxygen system under irradiation

“…In the early stages of oxidation at 280 °C, the only phase identified by XRD was U2N3+x, which was assumed to be a mixed oxynitride (U2N3+xOy [51]). At temperatures >400 °C, a UN/U2N3/UO2 sandwich structure formed with both the U2N3 and UO2 layers being epitaxially oriented with respect to the UN crystal [53].…”

“…Sesquinitride-U2N3 is body-centered cubic (bcc) with Mn2O3-type structure and ¾ of tetrahedral sites (total of eight) in -U2N3 sublattice are occupied by N atoms. Interstitial O atoms may enter into the unoccupied tetrahedral sites or replace N atoms in -U2N3 sublattice to form U2N3+xOy [51]. -U2N3 is hexagonal with a La2O3-type structure.…”

Nanoscale oxygen defect gradients in UO 2+ x surfaces

“…A major strength of scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS) approaches is that they provide high-resolution, simultaneous information about local structure, chemistry, and defects. Past studies have shown that STEM-EELS is capable of detecting minor changes in oxidation state and composition and that it compares favorably to X-ray results on similar uranium compounds (27–30). Within the dipole approximation, these results can also be modeled using first-principles methods, offering a means to quantify defect configurations and density (27, 31).…”

“…Past studies have shown that STEM-EELS is capable of detecting minor changes in oxidation state and composition and that it compares favorably to X-ray results on similar uranium compounds (27–30). Within the dipole approximation, these results can also be modeled using first-principles methods, offering a means to quantify defect configurations and density (27, 31). However, early STEM-EELS work on the actinides was performed using slower, lower-resolution spectrometers (32–37) and few studies (38, 39) have leveraged the advanced instrumentation or the supporting first-principles computing power developed in recent years.…”