Neutral Oxygen Research Articles

Ab initio calculations for the H-decorated neutral and charged oxygen vacancy in erbium oxide

Point defects created by neutron/electron irradiation often determine the performance and the chemical stability and activity of oxide materials. Oxygen vacancies constitute a common point defect in metal oxides, such as erbium oxide (Er2O3), one of the candidate materials for tritium permeation barriers in fusion blanket systems. However, a systematic study of the oxygen vacancy properties in Er2O3 has been lacking. Here, the properties of isolated neutral and charged oxygen vacancies in Er2O3 are investigated by means of supercell total-energy calculations using a first-principles method based on density–functional theory. Vacancy formation energies, hydrogen–vacancy interactions, and geometry rearrangements around these point defects are investigated in detail. The characterization of the electronic structure of these point defects is established by the analysis of the spin-orbital density of states. It is found that the energetic and electronic properties of the oxygen vacancies depend on the chemical potentials of the O and Er atoms in Er2O3. The defect formation energy decreases when one hydrogen atom is trapped into the vacancy in both charged oxidizing and reduced environments. Also, the interstitial hydrogen 1s orbital forms multicenter bonds with the 5d orbital of the neighboring Er atoms, and the net charge transfer from the defected Er2O3 is 1.04e, 1.02e, and 0.80e for the defect charge states of 1−, 0 and 1+, respectively. We suggest that the H-decorated vacancy defect may be responsible for the increase of the self-healing properties of Er2O3.

A comparative study of the point defect accumulation in the Ar+−implanted and neutron-irradiated silicon dioxide

The ion-implantation of silica is widely used in optoelectronics and neighboring areas. Si + ions serve for formation of clusters/nanocrystals in components of microelectronic devices; Ge+, P+, and B+ are used for controlling refraction index in lightguides. All enumerated ions interact chemically with oxygen atoms in SiO2, thus shading the stoichiometric distortions caused by direct impact interactions of particles with a host. In this research, the optically-active defects in the silica network were induced by embedding of chemically inert Ar + ions and thermal neutrons. A set of bands belonging to neutral oxygen vacancies (≡Si-Si≡), non-bridging oxygen hole centers (NBOHC) and some other point defects were detected in the photoluminescence spectra of implanted/irradiated samples. The intensity variation of the bands at the Ar + fluence increase was found to be caused by a competition between defect nucleation and their annealing or/and concentration quenching at high fluence. The ionization of atoms by charged particles resulted in the multivariant spectral manifestation of the defects of the same type due to their arrangement in differently distorted host localities. The diversification of defects produced by neutrons was lesser, because the neutron impact on the silica host was limited by the interactions with nuclei.

Effect of Mn in ZnO using DFT calculations: Magnetic and electronic changes

First principles calculations based on density functional theory (DFT) implemented in the VASP code was applied to study the structural, electronic and magnetic properties of ZnO and Mn-doped ZnO. Lattice parameters, bond lengths, energy band gap and magnetic moment were evaluated after the incorporation of neutral (VO0) and charged (VO2+) oxygen vacancies on both undoped and Mn-doped ZnO. Neutral oxygen vacancy produces a shrinkage of the distance to its nearest neighbor zinc atoms while the positive oxygen vacancy produces the opposite effect. The Zn–O bond length practically is not affected by incorporation of Mn atom, nevertheless, a magnetic moment is induced (5 μB) as new states at the band gap zone. The presence of oxygen vacancies strongly decreases magnetic moment for Mn-doped ZnO which could favor a transition from ferromagnetic to antiferromagnetic behaviour. These facts do the system Mn-doped ZnO with vacancies a promising candidate for designing the first antiferromagnetic piezoelectric material.

Functional Iron Oxide (Fe2O3) Nanoparticles Based Gas Sensor Operating at High Temperature

The highly responsive gas sensor based on a metal-oxide-semiconductor (MOS) configuration which is conformally coated with iron oxide (Fe2O3) nanoparticles is hereby designed, fabricated, and systematically characterized for this study. The iron oxide nanoparticles are prepared by mixing altogether the respective amounts of FeCl2-4H2O, FeCl3-6H2O, and NaOH in terms of different molar ratios. The iron oxide overcoated gas sensor thus fabricated has been electronically probed in an ecnclosed chamber under a high temperature condition during which the different gases are fed in individually and selectively. The presence of a perticular gas is duly sensed by monitoring the changing magnitude of the current extracted from the sensor. The enhanced sensitivity of the MOS-based sensor overlaid with the Fe2O3 nanoparticles is attributed to a large surface area typically associated with nanoscale particles. The fabrication of MOS-based sensor is entirely CMOS compatible and can be summarized as follows. First, the 400μm-thick silicon wafer is first RCA cleaned and then both sides of the wafer are coated with 800nm-thick oxide layer using high pressure oxidation furnace. Next, a series of photolithographic steps followed by electron-beam-evaporated titanium lift-off is implemented to realize the heater pattern. Afterward, the resultant heater pattern is overcoated with a 400nm-thick oxide layer using plasma-enhanced chemical vapor deposition (PECVD). In order to open up an area for the subsequent deposition of iron oxide nanoparticles-embedded material while leaving the titanium heater protected by the same oxide layer, necessary photolithographic and buffered oxide etching (BOE) steps are performed. Finally, the interdigitated electrodes are deposited followed by injecting the viscous Fe2O3 nanoparticles-embedded solution over the electrodes. Once the sensor is successfully fabricated, the very sensor is placed on a temperature-controlled sample platform maintained at a desired temperature within an enclosed chamber. The entire sensor characterization is performed at a temperature of 300 degree Celsius. The operating mechanism of the sensor is intimately dependent on the oxidation-reduction reaction between the injected gas and Fe2O3 nanoparticles-embedded sensing film. As can be shown in the follwoing figure, the ethanol (C2H5OH) gas vapor is functioned as a reducing agent, while nitrogen dioxide (NO2) gas, on the other hand, is behaved as a oxidizing agent instead. As ethanol gas vapor is injected, the oxygen ions of the ethanol produced at high temperature is converted back to the neutral oxygen atom while releasing the captured electrons back to to the iron oxide sensing film, thereby causing the detected current to go up. On the other hand, nitrogen dioxide is behaved as an oxidizing agent instead by oxidizing the Fe2O3 film and grabbing the electrons away, thereby causing the detected current to drop in response to the presence of the NO2 gas vapor. Consequently, the reversing current-versus-time patterns associated with C2H5OH and NO2, as shown in the follwing figure, are thus detected, which verify the different oxidation-reduction mechanisms associated with these two different gases. The optimization in the design of the iron oxide nanoparticles coated sensor in order to enhance the sensitivity and the response time in the detection of the different gases is currently being pursued and the subsequent experimental results obtained will be reported in the upcoming 235th ECS conference. Figure 1

Role of Oxygen Vacancy in Copper-Cobalt Spinel Nanostructures for Electrochemical Energy-Storage

The incorporation of oxygen vacancies in metal oxides has been reported to remarkably increase their electrochemical performance in water splitting, photoluminescence, and electrochemical energy-storage. The oxygen vacancies are originated by removal of neutral oxygen atoms from the metal oxides, leaving excess electrons in the vacancies. The excess electrons offer high electrical conductivity, huge number of electroactive sites at or within the surface of metal oxides that could facilitate a adsorption/intercalation of anions (e.g., OH-) at or within the electrode surface and diffusion of ions and electrons within the electrode/electrolyte interface for efficient energy-storage. It has been reported that the spinels with the proper oxygen vacancies could exhibit higher capacitance values compared with that for perfect-crystal. The agglomerated nanostructures of the spinels, Cu2Co2O4 are often prepared by conventional solid-phase or solvothermal/hydrothermal methods, which require non-conductive binders (e.g., Nafion and polyvinylidene fluoride (PVDF)) to achieve strong and stable adhesion onto a substrate. In addition, the conventional synthetic method usually requires high-temperature sintering with complicated procedures, which creates morphological deformation and severe aggregation, decreasing the content of electroactive oxygen vacancies. Therefore, the development of a synthetic procedure is one of the most important tasks needed to get effective nanostructures of Cu2Co2O4 with controllable surface morphology, oxygen vacancies/electroactive sites, and electrical conductivity for the application in capacitors as effective electrode materials. In the present report, we directly grew a series of CuxCo3 - xO4 nanostructures with controllable surface morphology, oxygen vacancies/electroactive sites and electrical conductivity on a stainless steel substrate by a simple and straightforward electrochemical deposition process and applied them as electrodes for electrochemical charge-storage.

Evidence of Europa Neutral Gas Torii From Energetic Sulfur Ion Measurements

AbstractImaging of energetic neutral atoms coming from a trans‐Europa region and energetic proton observations by Galileo around Jupiter have previously revealed the existence of a neutral gas torus produced by Europa. Energetic sulfur ions observed by Galileo do not show any effect of the Europa torus for Mc Ilwain parameters L > 9.2, surely because ions are multiply charged, so that the torus first needs to bring the ions charge state to one before being able to neutralize them. Signatures of the Europa torus in energetic sulfur ion measurements obtained near the magnetic equator at L = 9.08–9.20 during Galileo orbits C10 and E11 are reported for the first time in this letter. They provide an independent confirmation of the Europa neutral gas torus. Pitch angle distributions suggest that protons might principally interact with a vertically thin neutral oxygen torus, while sulfur ions are lost by charge exchange with a vertically thicker hydrogen torus.

Photoluminescence of Amorphous SiO2 with Implanted Ar+ Ions

Implantation of ions into amorphous silicon dioxide (a-SiO2) is widely used in microelectronics (mainly, using Si ions) and in the production of light-guiding components of optical fibers (Ge, P, B ions). All of these elements interact with the matrix oxygen, so the point defects that occur during implantation are not only the result of the displacement of the silicon–oxygen frame atoms from their equilibrium positions, but also reflect the specific chemical interaction in the material. In this study, inert argon ions were implanted into silicon dioxide plates, which excluded chemical reactions. It is demonstrated using photoluminescence (PL) that the highest concentration of point defects in the damaged silicate network belongs to neutral oxygen vacancies (NOVs). The concentration of these and some other induced defects increased with an increase in fluence up to 5 × 1015 Ar+/cm2, and, with a further increase in dose, the concentration dropped due to annealing of defects in the layer heated by the introduction of ions. The NOV defects appeared in the photoluminescence spectrum in the form of doublets, that is, pairs of bands belonging to the same optical transition. The appearance of doublets is explained by the dependence of the optical transition energy on the localization of identical point defects in zones of different distortions of the matrix structure.

Mechanisms of Oxygen Vacancy Aggregation in SiO2 and HfO2

Dielectric oxide films in electronic devices undergo significant structural changes during device operation under bias. These changes are usually attributed to aggregation of oxygen vacancies resulting in formation of oxygen depleted regions and conductive filaments. However, neutral oxygen vacancies have high diffusion barriers in ionic oxides and their interaction and propensity for aggregation are still poorly understood. In this paper we briefly review the existing data on static configurations of neutral dimers and trimers of oxygen vacancies in technologically relevant SiO$_2$ and HfO$_2$ and then provide new results on the structure and properties of these defects in amorphous SiO$_2$ and HfO$_2$. These results demonstrate weak interaction between neutral O vacancies, which does not explain their quick aggregation. We propose that trapping of electrons, injected from an electrode, by the vacancies may result in creation of new neutral vacancies in the vicinity of pre-existing vacancies. We describe this mechanism in a-SiO$_2$ and demonstrate that this process becomes more efficient as the vacancy clusters grow larger.

A first-principles study on magnetic properties of the intrinsic defects in wurtzite ZnO.

Since the origin of magnetism in ZnO-based diluted magnetic semiconductors (DMSs) is still controversial, in this work, we presented a detailed study on the magnetic, structural, and electronic properties of wurtzite ZnO-based DMS systems with point and complex intrinsic defects. Two outer electrons from neutral oxygen vacancy (VO) occupy the a1 orbital, making the inducted magnetic moment to be zero, while a cluster including three VOs leads to a magnetic moment of ∼1 μB. The magnetic moment of the system with a Zn vacancy (VZn) is 1.65 μB. When two neutral VZns in different relative distances were created in respective supercells, the systems showed different magnetic moments induced by the unequal level between the highest electron occupied orbital of the defect state introduced by different VZn sites and the valence band maximum. The system of a neutral O occupying an octahedral site gives rise to a magnetic moment of 2 μB, while zinc interstitial and antisite defects do not cause spin polarization. The system with a complex defect of VO and VZn is magnetic when those vacancies are adjacent but still do not cause the compensation effect. The oxygen interstitial defect is unstable, and VZn easily turns into the complex defect. We suggest that VO clusters and VZn complex defects could likely be the origin of ferromagnetism in undoped ZnO.

Oxygen fractionation in dense molecular clouds.

We have developed the first gas-grain chemical model for oxygen fractionation (also including sulphur fractionation) in dense molecular clouds, demonstrating that gas-phase chemistry generates variable oxygen fractionation levels, with a particularly strong effect for NO, SO, O2, and SO2. This large effect is due to the efficiency of the neutral 18O + NO, 18O + SO, and 18O + O2 exchange reactions. The modeling results were compared to new and existing observed isotopic ratios in a selection of cold cores. The good agreement between model and observations requires that the gas-phase abundance of neutral oxygen atoms is large in the observed regions. The S16O/S18O ratio is predicted to vary substantially over time showing that it can be used as a sensitive chemical proxy for matter evolution in dense molecular clouds.

Structural and optical properties of pulsed-laser deposited crystalline β-Ga2O3 thin films on silicon

Crystalline β-Ga2O3 thin films on (100)- and (111)-oriented Si substrates are produced by pulsed laser deposition. The as-deposited thin films are demonstrated to be polycrystalline and contain a slight deficit of oxygen atoms as measured by x-ray diffraction spectroscopy and Rutherford backscattering spectrometry, respectively. The crystallographic orientation of the Si substrate is found to play no role on the ultimate properties of the films. A direct optical band gap of 4.8 eV is determined by temperature-dependent photoluminescence excitation (PLE). Temperature-dependent PLE spectra reveal the existence of a deep acceptor level of around 1.1 eV with respect to the valence band related to self-trapped holes. We experimentally demonstrate that point defects in O-poor β-Ga2O3 thin films act as deep donors and the optical transitions are found to take place via recombination of electrons from one of the intrinsic deep donor levels with self-trapped holes located at 1.1 eV above the valence band. The 3.17 eV ultraviolet photoluminescence is proven to be related to self-trapped holes in a small polaron state between two O(II)-s sites, whereas the two blue (2.98, 2.72 eV) and the green (2.39 eV) luminescence bands are mainly originated from gallium-oxygen vacancy pairs in the (1-) charge state, gallium vacancies in the (2-) charge state and neutral oxygen interstitials, respectively.

Role of Nanostructuring on the Properties of Oxide Materials: The Case of Zirconia Nanoparticles

In this review we discuss how the geometric structures, the stability, the reactivity, and the magnetic properties of zirconia nanoparticles (NPs) may differ substantially from those of bulk zirconia or of the (101) ZrO2 surface. Based on extensive density functional calculations, we show that the formation energies of neutral oxygen vacancies are considerably smaller in zirconia NPs than those on the zirconia surface. The presence of low‐coordinated sites on the NPs generates defective states in the band gap of the material that results in enhanced chemical activity towards adsorbed molecular or atomic species, which also affects the physical properties. In particular, the following cases have been considered comparing zirconia bulk and NPs: (1) the adsorption of a Au atom; (2) the adsorption of a H2 molecule; (3) the reduction of zirconia via H2O desorption from a hydroxylated surface; (4) the appearance of high‐spin, magnetic ground states in reduced zirconia NPs. This provides evidence that nanostructuring can turn zirconia, a non‐reducible, non‐magnetic material, into a reducible magnetic oxide.

Ionization of gold (γ-methoxy)vinyl complexes generates reactive gold vinyl carbene complexes.

Cationic gold vinyl carbene/allylic cation complexes of the form (E)-[(L)AuC(H)C(H)CAr2]+ OTf- {L = IPr, Ar = Ph [(E)-5a], L = IPr, Ar = 4-C6H4OMe [(E)-5b], L = P(t-Bu)2 o-biphenyl, Ar = 4-C6H4OMe [(E)-5c]} were generated in solution via Lewis acid-mediated ionization of the corresponding gold (γ-methoxy)vinyl complexes (E)-(L)AuC(H)C(H)C(OMe)Ar2 at or below -95 °C. Complexes (E)-5b and (E)-5c were fully characterized in solution employing multinuclear NMR spectroscopy, which established the predominant contribution of the aurated allylic cation resonance structure and the significant distribution of positive charge into the γ-anisyl rings. Complex (E)-5b reacted rapidly at -95 °C with neutral two-electron, hydride, and oxygen atom donors exclusively at the C1 position of the vinyl carbene moiety and with p-methoxystyrene to form the corresponding vinylcyclopropane. In the absence of nucleophile (E)-5a decomposed predominantly via intermolecular carbene dimerization whereas formation of 1-aryl-5-methoxy indene upon ionization of (Z)-(IPr)AuC(H)C(H)C(OMe)(4-C6H4OMe)2 [(Z)-6b] implicated an intramolecular Friedel-Crafts or electrocyclic Nazarov pathway for the decomposition of the unobserved vinyl carbene complex (Z)-[(IPr)AuC(H)C(H)C(4-C6H4OMe)2]+ OTf- [(Z)-5b].

Фотолюминесценция аморфного SiO-=SUB=-2-=/SUB=-, подвергнутого имплантации ионов Ar-=SUP=-+-=/SUP=-

AbstractImplantation of ions into amorphous silicon dioxide ( a -SiO_2) is widely used in microelectronics (mainly, using Si ions) and in the production of light-guiding components of optical fibers (Ge, P, B ions). All of these elements interact with the matrix oxygen, so the point defects that occur during implantation are not only the result of the displacement of the silicon–oxygen frame atoms from their equilibrium positions, but also reflect the specific chemical interaction in the material. In this study, inert argon ions were implanted into silicon dioxide plates, which excluded chemical reactions. It is demonstrated using photoluminescence (PL) that the highest concentration of point defects in the damaged silicate network belongs to neutral oxygen vacancies (NOVs). The concentration of these and some other induced defects increased with an increase in fluence up to 5 × 10^15 Ar^+/cm^2, and, with a further increase in dose, the concentration dropped due to annealing of defects in the layer heated by the introduction of ions. The NOV defects appeared in the photoluminescence spectrum in the form of doublets, that is, pairs of bands belonging to the same optical transition. The appearance of doublets is explained by the dependence of the optical transition energy on the localization of identical point defects in zones of different distortions of the matrix structure.

First principle study of formation mechanism of molybdenum-doped amorphous silica in MoO<sub>3</sub>/Si interface

An amorphous mixing layer (3.5–4.0 nm in thickness) containing silicon (Si), oxygen (O), molybdenum (Mo) atoms, named <i>α</i>-SiO<sub><i>x</i></sub>(Mo), is usually formed by evaporating molybdenum trioxide (MoO<sub>3</sub>) powder on an n-type Si substrate. In order to investigate the process of adsorption, diffusion and nucleation of MoO<sub>3</sub> in the evaporation process and ascertain the formation mechanism of <i>α</i>-SiO<sub><i>x</i></sub>(Mo) on a atomic scale, the first principle calculation is used and all the results are obtained by using the Vienna <i>ab initio</i> simulation package. The possible adsorption model of MoO<sub>3</sub> on the Si (100) and the defect formation energy for substitutional defects and vacancy defects in <i>α</i>-SiO<sub>2</sub> and <i>α</i>-MoO<sub>3</sub> are calculated by the density functional theory. The results show that an amorphous layer is formed between MoO<sub>3</sub> film and Si (100) substrate according to <i>ab initio</i> molecular dynamics at 1500 K, which are in good agreement with experimental observations. The O and Mo atoms diffuse into Si substrate and form the bonds of Si—O or Si—O—Mo, and finally, form an <i>α</i>-SiO<sub><i>x</i></sub>(Mo) layer. The adsorption site of MoO<sub>3</sub> on the reconstructed Si (100) surface, where the two oxygen atoms of MoO<sub>3</sub> bond with two silicon atoms of Si (100) surface, is the most stable and the adsorption energy is －5.36 eV, accompanied by the electrons transport from Si to O. After the adsorption of MoO<sub>3</sub> on the Si substrate, the structure of MoO<sub>3</sub> is changed. Two Mo—O bond lengths of MoO<sub>3</sub> are 1.95 Å and 1.94 Å, respectively, elongated by 0.22 Å and 0.21 Å compared with the those before the adsorption of MoO<sub>3</sub> on Si substrate, while the last bond length of MoO<sub>3</sub> is little changed. The defect formation energy value of neutral oxygen vacancy in <i>α</i>-SiO<sub>2</sub> is 5.11 eV and the defect formation energy values of neutral oxygen vacancy in <i>α</i>-MoO<sub>3</sub> are 0.96 eV, 1.96 eV and 3.19 eV, respectively. So it is easier to form oxygen vacancy in MoO<sub>3</sub> than in SiO<sub>2</sub>, which implies that the oxygen atoms will migrate from MoO<sub>3</sub> to SiO<sub>2</sub> and forms a 3.5–4.0-nm-thick <i>α</i>-SiO<sub><i>x</i></sub>(Mo) layer. As for the substitutional defects in MoO<sub>3</sub> and SiO<sub>2</sub>, Mo substitutional defects are most likely to form in SiO<sub>2</sub> in a large range of Mo chemical potential. So based on our obtained results, the forming process of the amorphous mixing layer may be as follows: the O atoms from MoO<sub>3</sub> bond with Si atoms first and form the SiO<sub><i>x</i></sub>. Then, part of Mo atoms are likely to replace Si atoms in SiO<sub><i>x</i></sub>. Finally, the ultra-thin buffer layer containing Si, O, Mo atoms is formed at the interface of MoO<sub>3</sub>/Si. This work simulates the reaction of MoO<sub>3</sub>/Si interface and makes clear the interfacial geometry. It is good for us to further understand the process of adsorption and diffusion of atoms during evaporating, and it also provides a theoretical explanation for the experimental phenomenon and conduces to obtaining better interface passivation and high conversion efficiency of solar cell.

Neutral Oxygen Beam Treated ZnO-Based Resistive Switching Memory Device

The room-temperature oxidation process allows irradiation with neutral oxygen particles onto the resistive layer that leads to the absorption of oxygen by the surface of the ZnO layer. The irradiation is effective in controlling the defect concentrations; thus, the ON and OFF resistances of devices can be significantly increased. These characteristics promote the occurrence of resistive switching at much lower current compliance as well as induce switching behavior in very thin ZnO films with thicknesses of 14–42 nm. The thickness dependence of the transformation from filamentary to homogeneous switching was also studied using the neutral beam technique, and the underlying mechanism is discussed.

SOFIA-HIRMES: Looking Forward to the HIgh-Resolution Mid-infrarEd Spectrometer

The HIgh-Resolution Mid-infrarEd Spectrometer (HIRMES) is the 3rd Generation Instrument for the Stratospheric Observatory For Infrared Astronomy (SOFIA), currently in development at the NASA Goddard Space Flight Center (GSFC), and due for commissioning in 2019. By combining direct-detection Transition Edge Sensor (TES) bolometer arrays, grating-dispersive spectroscopy, and a host of Fabry-Perot tunable filters, HIRMES will provide the ability for high resolution ([Formula: see text]), mid-resolution ([Formula: see text]), and low-resolution ([Formula: see text]) slit-spectroscopy, and 2D Spectral Imaging ([Formula: see text] at selected wavelengths) over the 25–122[Formula: see text][Formula: see text]m mid to far infrared waveband. The driving science application is the evolution of proto-planetary systems via measurements of water-vapor, water-ice, deuterated hydrogen (HD), and neutral oxygen lines. However, HIRMES has been designed to be as flexible as possible to cover a wide range of science cases that fall within its phase-space, all whilst reaching sensitivities and observing powers not yet seen thus far on SOFIA, providing unique observing capabilities which will remain unmatched for decades.

Oxygen vacancies in the bulk and at neutral domain walls in hexagonal YMnO3

We use density functional calculations to investigate the accommodation and migration of oxygen vacancies in bulk hexagonal YMnO$_3$, and to study interactions between neutral ferroelectric domain walls and oxygen vacancies. Our calculations show that oxygen vacancies in bulk YMnO$_3$ are more stable in the Mn-O layers than in the Y-O layers. Migration barriers of the planar oxygen vacancies are high compared to oxygen vacancies in perovskites, and to previously reported values for oxygen interstitials in h-YMnO$_3$. The calculated polarization decreases linearly with vacancy concentration, while the out-of-plane lattice parameter expands in agreement with previous experiments. In contrast with ferroelectric perovskites, oxygen vacancies are found to be more stable in bulk than at domain walls. The tendency of oxygen vacancies to segregate away from neutral domain walls is explained by unfavorable Y-O bond lengths caused by the local strain field at the domain walls.

First Principles Simulations on Migration Paths of Oxygen Interstitials in MgAl2O4

Thermal stability of the primary electronic defects – F‐type centers – in oxide materials is controlled by their recombination with much more mobile complementary defects – interstitial oxygen ions Oi. Thus, the study of interstitial ion migration is of key importance for the prediction of radiation damage in oxides. In this study, several possible migration trajectories for neutral and charged interstitial oxygen ions are calculated in MgAl2O4spinel using the first principles calculations of atomic and electronic structure. The lowest energy barriers are ≈1.0–1.1 eV and 0.8 eV, respectively. The effective atomic charges, charge redistribution, and lengths of bonds closest to Oiinterstitials are analyzed in detail.

Hole accumulation effect on Laser-assisted field evaporation of insulators

Current issues associated with laser-assisted atom probe tomography of insulators are addressed by investigating laser-induced carrier dynamics and field evaporation kinetics. It is shown that for typical insulators with slow carrier recombination compared to the sub-picosecond laser pulse, hole accumulation at the surface plays a key role. By carrying out density functional theory calculations on a MgO cluster, it is found that the critical evaporation field strength decreases linearly as the surface hole density increases. This phenomenon can be explained by the hole-induced electric field. The evaporation of neutral oxygen is enhanced at low electrostatic field strength and high laser intensity. Theoretical insight is also provided for the non-stoichiometry problem in the mass spectra measured in atom probe tomography of compounds.