Octahedral Interstitial Sites Research Articles

Strong nonlinear-polarization in ZnMgO epitaxial thin-films with Li incorporation

The second-order nonlinear-polarization originated from the interaction between thin-film materials with second-order nonlinear susceptibility (χ (2)) and high-power laser is essential for integrated optics and photonics. In this work, strong second-order nonlinear-polarization was found in a-axis oriented Zn1-x Mg x O (ZnMgO) epitaxial thin-films with Li incorporation, which were deposited by radio-frequency magnetron sputtering. Mg incorporation (x > 0.3) causes a sharp fall in the matrix element χ 33 of χ (2) tensor, although it widens optical bandgap (E opt). In contrast, moderate Li incorporation significantly improves χ 33 and resistance to high-power laser pulses with a little influence on E opt. In particular, a Zn0.67Mg0.33O:Li [Li/(Zn + Mg + Li) = 0.07] thin-film shows a |χ 33| of 36.1 pm V−1 under a peak power density (E p) of 81.2 GW cm−2, a resistance to laser pulses with E p of up to 124.9 GW cm−2, and an E opt of 3.95 eV. Compared to that of ZnO, these parameters increase by 37.8%, 53.4%, and 18.6%, respectively. Specially, the Zn0.67Mg0.33O:Li shows higher radiation resistance than a Mg-doped LiNbO3 crystal with a comparable E opt. First-principle calculations reveal the Li occupation at octahedral interstitial sites of wurtzite ZnO enhances radiation resistance by improving structural stability. X-ray photoelectron spectroscopy characterizations suggest moderate Li incorporation increases χ 33 via enhancing electronic polarization. These findings uncover the close relationship between the octahedra interstitial defects in wurtzite ZnMgO and its nonlinear-polarization behavior under the optical frequency electric field of high-power laser.

Hydrogen-Stabilized ScYNdGd Medium-Entropy Alloy for Hydrogen Storage.

The research on the functional properties of medium- and high-entropy alloys (MEAs and HEAs) has been in the spotlight recently. Many significant discoveries have been made lately in hydrogen-based economy-related research where these alloys may be utilized in all of its key sectors: water electrolysis, hydrogen storage, and fuel cell applications. Despite the rapid development of MEAs and HEAs with the ability to reversibly absorb hydrogen, the research is limited to transition-metal-based alloys that crystallize in body-centered cubic solid solution or Laves phase structures. To date, no study has been devoted to the hydrogenation of rare-earth-element (REE)-based MEAs or HEAs, as well as to the alloys crystallizing in face-centered-cubic (FCC) or hexagonal-close-packed structures. Here, we elucidate the formation and hydrogen storage properties of REE-based ScYNdGd MEA. More specifically, we present the astounding stabilization of the single-phase FCC structure induced by the hydrogen absorption process. Moreover, the measured unprecedented high storage capacity of 2.5 H/M has been observed after hydrogenation conducted under mild conditions that proceeded without any phase transformation in the material. The studied MEA can be facilely activated, even after a long passivation time. The results of complementary measurements showed that the hydrogen desorption process proceeds in two steps. In the first, hydrogen is released from octahedral interstitial sites at relatively low temperatures. In the second, high-temperature process, it is associated with the desorption of hydrogen atoms stored in tetrahedral sites. The presented results may impact future research of a novel group of REE-based MEAs and HEAs with adaptable hydrogen storage properties and a broad scope of possible applications.

Vacancy-enhanced oxidation resistance of Fe2CrAl high-temperature alloy

Although Fe2CrAl ternary alloy is a promising high temperature material, the oxidation mechanism of Fe2CrAl is unknown. To solve the problem, here, we apply the first-principles method to study the oxidized behavior of Fe2CrAl ternary alloy. In particular, the influence of vacancies on the oxidation resistance of Fe2CrAl is further studied. The calculated results shows that the oxygen prefers to occupy the octahedral interstitial (OI) site because of the interaction of O-Al, O-Cr and O-Fe atoms. It is found that these vacancies are thermodynamic stability in Fe2CrAl ternary alloy. In particular, the Cr-vacancy has better thermodynamic stability compared to the Fe-vacancy and Al-vacancy. Importantly, the Cr-vacancy has strongest the oxidation resistance because the Cr-vacancy enhances the localized hybridization between O atom and Al atom, which form the Al2O3 oxide. The formation of Al2O3 is demonstrated by the bond length of Al-O bond in Al2O3. In addition, the calculated elastic modulus of the oxidized Fe2CrAl is lower than the unoxidized Fe2CrAl whether the parent Fe2CrAl or Fe2CrAl with three vacancies. However, the Fe2CrAl with Cr-vacancy retains the high elastic stiffness compared to the Fe-vacancy and Al-vacancy. Therefore, we believe the Cr-vacancy is beneficial to improve the oxidation resistance of Fe2CrAl ternary alloy.

Hydrogen storage of high entropy alloy NbTiVZr and its effect on mechanical properties: A first-principles study

In recent years, high entropy alloys (HEAs) have demonstrated remarkable potential for hydrogen storage applications. Although extensive experimental studies have been conducted, a detailed understanding of the hydrogenation process at the atomic level is still lacking. In this study, first-principles calculations were employed to explore the microstructural evolution during hydrogen absorption in NbTiVZr as well as the mechanical characteristics of hydrides. The results indicate that a phase transition from BCC to FCC occurs in the hydride when the hydrogen content reaches 0.05 wt%. Hydrogen tends to occupy the octahedral interstitial sites in the BCC hydrides, while the preferred hydrogen sites in FCC hydrides undergo a transition from octahedral sites → tetrahedral + octahedra sites → tetrahedral sites as the hydrogen content increases. The highest hydrogen storage capacity of NbTiVZr was predicted by the phonon spectra of hydrides to be 2.94 wt%. Vanadium (V) is discovered to play a crucial role in hydrogen absorption capacity by causing significant lattice distortion and forming stronger bonds with hydrogen. Additionally, all hydrides exhibit great mechanical properties and thermal stability. Our research reveals that NbTiVZr has an excellent capacity for storing hydrogen and has the potential for applications in hydrogen storage materials.

Near Room-Temperature Intrinsic Exchange Bias in an Fe Intercalated ZrSe2 Spin Glass.

Some magnetic systems display a shift in the center of their magnetic hysteresis loop away from zero field, a phenomenon termed exchange bias. Despite the extensive use of the exchange bias effect, particularly in magnetic multilayers, for the design of spin-based memory/electronics devices, a comprehensive mechanistic understanding of this effect remains a longstanding problem. Recent work has shown that disorder-induced spin frustration might play a key role in exchange bias, suggesting new materials design approaches for spin-based electronic devices that harness this effect. Here, we design a spin glass with strong spin frustration induced by magnetic disorder by exploiting the distinctive structure of Fe intercalated ZrSe2, where Fe(II) centers are shown to occupy both octahedral and tetrahedral interstitial sites and to distribute between ZrSe2 layers without long-range structural order. Notably, we observe behavior consistent with a magnetically frustrated and multidegenerate ground state in these Fe0.17ZrSe2 single crystals, which persists above room temperature. Moreover, this magnetic frustration leads to a robust and tunable exchange bias up to 250 K. These results not only offer important insights into the effects of magnetic disorder and frustration in magnetic materials generally, but also highlight as design strategy the idea that a large exchange bias can arise from an inhomogeneous microscopic environment without discernible long-range magnetic order. In addition, these results show that intercalated TMDs like Fe0.17ZrSe2 hold potential for spintronic technologies that can achieve room temperature applications.

Linear-scaling density functional theory (DFT) simulations of point, Frenkel and Schottky defects in CeO2

CeO2 (ceria) is a material of significant industrial and technological importance, used in solid oxide fuel cells and catalysis. Here, we explore the usage of linear-scaling density functional theory as implemented in the ONETEP code, which allows to use larger simulation cells. By using DFT+U calculations we revise the defect chemistry of ceria, including point defects, Frenkel and Schottky defects.We found that the ground state of an oxygen vacancy is associated to two neighbouring reduced cerium sites. A cerium vacancy is the least favourable point defect, where holes localise on neighbouring oxygen sites. It is more favourable to displace an oxygen interstitial defect away from the octahedral interstitial site, with the formation of a stable peroxide species. Our simulations show that a cerium interstitial is best accommodated in the octahedral interstitial site, as this minimises the distortion of the lattice.Placing a vacancy and an interstitial defect at a separation of 5.18 Å for the OF¡110¿ and 4.77 Å for the CeF¡111¿, stable Frenkel defects can be formed. We also studied the effect of different supercell size on the energetic ordering of Schottky defects, where the S¡111¿ is more favourable than the S¡110¿ for a given simulation cells containing 324 or more atoms.

First-principles investigation of hydrogen interaction with Cu/WC interface

The WC-Cu composite used as a thermal barrier interlayer between tungsten and CuCrZr heat sink can effectively alleviate the thermal strain mismatch. In this work, the interaction of hydrogen (H) and Cu/WC interface are investigated by performing the first-principles density functional theory calculations. Considering the H solubility in the WC bulk, we found that the most energetically favorable site is not the traditional octahedral interstitial site but is the projection site of octahedral interstitial on the W basal plane. The H diffusion barrier for octahedral interstitial sites and the diffusion path refers to the channel for H penetration and migration into the bulk WC is as high as 0.773 eV. According to the interface calculations, it indicated that the C-terminated interface with the atoms in WC followed the stacking sequence of the Cu (111) plane can yield the strong adhesion for (111)Cu/(0001)WC interface. Furthermore, the higher dissolution energy for H in this Cu/WC interface indicates no interstitial segregation or trapping site for H in the region due to the localization of interface electrons. Note that the H atom in WC also exhibits a high diffusion barrier. Both of these factors combine to determine that the interface can effectively block the penetration behavior of H into the WC, and the WC-Cu composite materials can use as the H permeation barrier. Besides, our ab initio molecular dynamics (AIMD) simulation also confirmed that H positioned at the interface can spontaneously migrate from the interface to the Cu matrix, and the interface can block the H penetration.

Effect of reaction path on high-pressure synthesis and stability of ruthenium hydrides

This study explores the behavior of ruthenium hydrides under high-pressure conditions through three thermodynamical paths using laser-heated diamond anvil cells. The synthesis of RuH0.9 occurs gradually exceeding the pressure of 23.5 GPa in the ambient temperature path, while RuH is successfully synthesized at pressures above 20 GPa and a temperature of 1500 K. High-temperature conditions are found to reduce the pressure required for synthesis. The results demonstrate that the hydrogen occupancy of octahedral interstitial sites in the ruthenium hydrides is found to reach saturation with complete hydrogen absorption in the high-temperature path. Moreover, the crystallinity of the ruthenium hydride samples improves at higher temperatures, with the grain size increasing from 10 nm in the ambient temperature path to submicron in the high-temperature path. However, the predicted RuH6 and RuH3 were not observed in the present work.

Atomic structure and annealing-induced reordering of ε-Ga2O3: A Rutherford backscattering/channeling and spectroscopic ellipsometry study

The crystallographic structure of thin Ga2O3 layers grown by metal–organic vapour phase epitaxy on Al2O3 substrate was analyzed by Rutherford Backscattering Spectrometry/Channeling (RBS/C) angular yield scans performed around the c-axis of as-grown Ga2O3. The measured widths and minimum yields of the scan curves for the Ga and O component were compared to calculations based on the continuum steering potential model. The results obtained are consistent with a crystal structure containing oxygen atoms arranged in a 4H hexagonal closely packed lattice and Ga atoms preferentially occupying octahedral interstitial sites in the 4H cells - a structure closely related to the ε-Ga2O3 polymorph. After high-temperature annealing remarkable structural transformation is detected via significant changes in the RBS/C spectra. This effect is related to the hexagonal-monoclinic, i.e., ε-β phase transformation of Ga2O3. Spectroscopic ellipsometry spectra of as-grown and annealed samples can be best fitted using a vertically graded single-layer B-spline model. Significant differences in the dielectric functions were found, showing bandgap reduction for long term annealing. These features are related to the ε-β polymorphic transformation, variation of the preferred crystallographic orientation upon annealing, and differences in residual strain and defect structure determined by the annealing conditions.

Enhancement in the electromagnetic shielding properties of doped [formula omitted] magnetite nanoparticles [formula omitted

Fe3O4 nanoparticles (NPs) are one of the most investigated materials in the field of microwave absorbers. To further optimize the electromagnetic interference (EMI) shielding effectiveness, doped M0.01Fe2.99O4(M=Mn,Ni,Cu,Zn) NPs were synthesized through co-precipitation method. All samples were subjected to X-ray diffraction analysis to determine their crystal structure. Transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy were performed to confirm the morphologies, distribution of elements, and valence of iron in the prepared nanoparticles, respectively. The dielectric and magnetic characteristics displayed a considerable improvement for the doped Fe3O4 samples, except Zn0.01Fe2.99O4 where the obtained values were found to be comparable to those of pure Fe3O4. These variations are described in terms of the homogeneously distributed microstructure, mixed spinel cationic distribution in the doped samples, and Fe2+/Fe3+ ratios at the octahedral interstitial sites. Moreover, the EMI shielding efficiency was measured in the X-band frequency range (8.2 – 12.4 GHz) using a wave guide approach. The total shielding effectiveness of Cu0.01Fe2.99O4 and Ni0.01Fe2.99O4 samples was found to be much higher than that of pure Fe3O4 with absorption being the dominant mechanism. This is most likely owing to their high dielectric permittivity, enhanced conductivity, and high magnetic permeability, which results in improved impedance matching and stronger microwave absorption. This suggests that these doped nanoparticles may be utilized as efficient electromagnetic absorbers.

Modification mechanism of Ti-6Al-4V alloy with pre-coated Ti-Cu-Al multilayer film treated by ion nitriding: Experiments and first-principles calculations

In this paper, the Ti-6Al-4V(TC4) alloy specimens pre-coated with Ti-Cu-Al multilayer films were plasma nitrided at different temperatures (673K-823K), and their microstructure and mechanical properties were studied. Surface hardness changes induced by pre-coating combined with plasma nitriding were also analyzed and correlated with microstructural features and elemental distribution. Ti-Cu-Al multilayer films with clear interfaces on deformed TC4 alloy were fabricated. The multilayer structure survived after nitriding, and a thick gradient nitrided layer with a gradient distribution of N elements was formed. The bonded nitride on the surface provides high hardness, reaching 948HV at 723K. In addition, the nitride TC4 alloy's wear resistance exhibits a significant improvement compared with the untreated specimen, especially at 823K, the mass wear rate decreased by more than 80%. It is mainly attributed to the N/metal element gradient distribution of the thick nitride layer. The nitriding behavior of α-Ti/Cu/Al was investigated by first-principles calculations. According to the thermodynamic calculation results, N atoms preferentially occupy the octahedral interstitial sites of α-Ti and the tetrahedral interstitial sites of Al. It is worth noting that N atoms have poor thermodynamic stability in Cu, which is consistent with the experimental results. The calculated elastic properties agree with the experimental cross-sectional hardness results, showing a smooth decreasing trend of the complex modulus and hardness on the micrometer length scale and explaining the improved structure performance. These results may provide crucial inspirations for future experimental work preparing high-performance nitrided layers.

Interstitial elements created via metal 3D printing

3D printing of metals, such as laser powder bed fusion (LPBF) printing of stainless steels, often leads to elevated oxygen content in the alloy substrate relative to conventional processing routes. Here we show that the extremely rapid cooling rate (106–107 K/s) during LPBF processing of austenitic stainless steel can trap a considerable fraction of oxygen and other elements in the interstitial sites of the metal lattice, at concentrations far exceeding the room temperature solubilities. High resolution characterization and atomistic simulations with density functional theory reveal that oxygen and other elements exist in octahedral interstitial sites of the metal lattice and bond with their neighboring metal atoms. Our findings suggest that additive manufacturing can be a potential strategy of incorporating beneficial interstitial elements into a metal substrate. Given the well-known effect of interstitial elements in conventional alloys, significant improvement of the physicochemical properties of printed alloys is possible.

Investigation on the effect of carbon on clustering of defects in tungsten by first principles calculations

Carbon (C) impurity would be inevitably introduced in the process of industrial-scale tungsten production. In this work, by using the first-principles density functional theory, the C atom behaviors in tungsten lattice and the effect of C atom on defects clustering have been studied systematically. Calculation results show that C atom prefers to occupy the octahedral interstitial site of the tungsten lattice, the migration barrier energy of C atom is 1.46 eV along the optimal pathway. Both vacancy cluster and C atom can form stable complex structure according to the attractive interaction, and C atom promotes the nucleation of the vacancy cluster. The growth mechanism shows that vacancy clusters are more likely to grow by combining a cluster rather than a single vacancy, where C atom plays a positive role of promoting vacancy clusters nucleation. In addition, C atom can cause tungsten lattice distortion and lead to the formation of the potential well, which prompts SIA rapidly diffuse toward it and forms a SIA-C complex structure. The calculation results of the nucleation rate indicate that C atom can enlarge the effective radius of the 2SIA cluster, and enhance the nucleation rate of 3SIA cluster. Therefore, we conjecture that the C atom can inhibit the recovery of the Frenkel pair and weaken the self-repair of the tungsten materials under irradiation. These results are meaningful for further understanding the mechanism of the effect of C impurity for the defects evolution of irradiation defects of W-based material.

Dislocation-twin interaction in medium entropy alloy containing a high density of oxygen interstitials

Dislocation-twin interactions play important roles in tuning the plastic deformation of materials. Here, we report the great influence of interstitial oxygen atoms on dislocation-twin interactions in NiCoCrO medium entropy alloy through in-situ transmission electron microscopy mechanical testing, atomic resolution energy-dispersive X-ray spectroscopy characterizations, and integrated differential phase-contrast scanning transmission electron microscopy imaging. The spreading lattice distortion in medium-entropy alloy was found to enable large oxygen interstitial solubility. Oxygen atoms occupied both the tetrahedral and octahedral interstitial sites, and tended to be enriched at defects such as twin boundaries and stacking faults. Oxygen-enriched regions near the twin boundary hindered dislocations from direct transmitting or cross-slip into another twin plane during plastic deformation, resulting in significant dislocation entanglement, partial dissociation of dislocations, and the deviation of slip directions. Our results give insight into the strengthening mechanisms associated with the synergetic effects of twin boundary and interstitial atoms.

The interaction between a positive muon and multiple quadrupolar nuclei

A positively charged muon implanted in copper sits at an octahedral interstitial site and experiences a magnetic dipolar coupling with six nearest-neighbour quadrupolar I = 3/2 copper nuclei. The resulting avoided level crossing resonance observed as a function of magnetic field provides a means of studying these interactions and understanding the effect of the electric-field gradient due to the muon acting on the quadrupolar nuclei. The effect is usually modelled by considering the interaction between the positive muon and a single copper nucleus, but the other five copper nuclei are equally important. By solving the problem in the full 2(2I + 1)6 = 8192-dimensional Hilbert space, we demonstrate the effect of these additional interactions.

The Site and High Field βNMR Properties of 8Li+ Implanted in α-Al2O3

We present high (> 2 T) magnetic field βNMR measurements of 8Li+ implanted in single crystals of sapphire, a commonly used backing material for other samples. From the well-resolved quadrupolar splitting, we extract the electric field gradient (EFG) at the implanted 8Li+ site. Comparison with supercell density functional theory calculations of the EFG allows us to identify the octahedral interstitial site as the most likely candidate. In contrast to the zero field βNQR spectra, where multiple singals are detected, only a single site is evident at high field. We discuss possible explanations for this discrepancy. The spin lattice relaxation is extremely slow (1/T 1 < 0.02 s−1 ) over a broad temperature range from 4 to 300 K, demonstrating that cross relaxation with the 27Al nuclear spins is quenched in such high magnetic fields.

Microscopic Mechanism of Enhanced Catalytic Activity for Ammonia Synthesis in Y5M3(M = Si/Ge) Electrides

Due to the importance of ammonia in modern agriculture, industry, scientific research, and so on, exploring a high-performance catalyst for ammonia synthesis at mild conditions is always highly desirable and will continuously be a hot research area. In recent years, electride catalysts have drawn considerable attention due to the presence of excess interstitial electrons. Among the known electride catalysts, the electride Y5Si3 is particularly interesting owing to its excellent catalytic activity in both air and water conditions. However, the underlying microscopic mechanism for the whole reaction process of the ammonia synthesis on the Y5Si3 surface remains unclear, in particular for the role played by the interstitial electrons. Here, through first-principles calculations, we have systematically studied the adsorption structures and energies of an atom (H and N) or a molecule (H2, N2, and NH3), the dissociation of H2 and N2 molecules, and the energy barriers for the formation of intermediate complexes such as NH and NH2 and the final NH3 on the Y5Si3(0001) surface. Our charge density analysis unambiguously demonstrates that the surface interstitial electrons play a very important role in facilitating the N2 dissociation as well as the formation of NH3 by providing necessary electrons that reduce the energy barriers. Moreover, we have carried out a comparative study on another isostructural and isoelectronic electride Y5Ge3. We find that the energy barriers in the rate-determining step on both electride surfaces are lower than that on Ru(0001), suggesting their good catalytic activities in synthesizing NH3. We further show that the conclusion remains unchanged by the presence of a N atom occupying the sublayer octahedral interstitial site.

First principles investigation of trapping and diffusion of fission gas in AnO2

The trapping of fission gases in AnO2 has been investigated using density functional theory. The behavior of fission gases varies with the size in these oxides, therefore He and Xe have been considered in this work. In ThO2 and UO2, the octahedral interstitial site has been found to be more stable than oxygen vacancy, while actinide vacancy is found most stable site for He in all three oxides. For Xe, an oxygen vacancy is a more stable site than an octahedral interstitial site and actinide vacancy is the most stable site in these oxides. Our results indicate that He is much more mobile than Xe in these oxides.[copyright information to be updated in production process]

Nanoscale Iron Redistribution during Thermochemical Decomposition of CaTi1-x Fe x O3-δ Alters the Electrical Transport Pathway: Implications for Oxygen-Transport Membranes, Electrocatalysis, and Photocatalysis.

Potential applications of the earth-abundant, low-cost, and non-critical perovskite CaTi1-x Fe x O3-δ in electrocatalysis, photocatalysis, and oxygen-transport membranes have motivated research to tune its chemical composition and morphology. However, investigations on the decomposition mechanism(s) of CaTi1-x Fe x O3-δ under thermochemically reducing conditions are limited, and direct evidence of the nano- and atomic-level decomposition process is not available in the literature. In this work, the phase evolution of CaTi1-x Fe x O3-δ (x = 0-0.4) was investigated in a H2-containing atmosphere after heat treatments up to 600 °C. The results show that CaTi1-x Fe x O3-δ maintained a stable perovskite phase at low Fe contents while exhibiting a phase decomposition to Fe/Fe oxide nanoparticles as the Fe content increases. In CaTi0.7Fe0.3O3-δ and CaTi0.6Fe0.4O3-δ, the phase evolution to Fe/Fe oxide was greatly influenced by the temperature: Only temperatures of 300 °C and greater facilitated phase evolution. Fully coherent Fe-rich and Fe-depleted perovskite nanodomains were observed directly by atomic-resolution scanning transmission electron microscopy. Prior evidence for such nanodomain formation was not found, and it is thought to result from a near-surface Kirkendall-like phenomenon caused by Fe migration in the absence of Ca and Ti co-migration. Density functional theory simulations of Fe-doped bulk models reveal that Fe in an octahedral interstitial site is energetically more favorable than in a tetrahedral site. In addition to coherent nanodomains, agglomerated Fe/Fe oxide nanoparticles formed on the ceramic surface during decomposition, which altered the electrical transport mechanism. From temperature-dependent electrical conductivity measurements, it was found that heat treatment and phase decomposition change the transport mechanism from thermally activated p-type electronic conductivity through the perovskite to electronic conduction through the iron oxide formed by thermochemical decomposition. This understanding will be useful to those who are developing or employing this and similar earth-abundant functional perovskites for use under reducing conditions, at elevated temperatures, and when designing materials syntheses and processes.

The interaction of oxygen with the γ-U (001) and (110) surfaces: An ab initio study

The reactions of the γ-U (001) and (110) surfaces in contact with O2 were investigated by density functional theory (DFT), ab initio thermodynamic methods and ab initio molecular dynamics simulations (AIMD). The adsorption of the O atom and O2 molecule on the (001) and (110) surfaces and subsurface are first studied, which provides insights into the adsorption, dissociation, and diffusion of O2 on the surfaces. Furthermore, the effects of coverages on surface adsorption are considered to help us further understand surface oxidation corrosion. Based on the above research, we can speculate on the initial process of surface corrosion: 1. O2 molecules in the atmosphere first approach the U surface in the form of physical adsorption; 2. O2 molecules dissociate into O atoms on the surface; 3. O atoms diffuse on the surface after overcoming a small energy barrier and eventually occupy hollow (H) sites; 4. O atoms penetrate from the H sites to the subsurface; 5. The diffusion of O atoms inside the metal is more likely to be carried out by octahedral interstitial sites. Moreover, the adsorption phase diagrams indicate that O2 will readily react with U even under ultra-high vacuum conditions, which is consistent with the experimental results.