Nearest-neighbor Oxygen Atoms Research Articles

73Ge, 17O and 29Si hyperfine interactions of the [formula omitted] center in crystalline SiO2

E′ centers as fundamental radiation-induced defects in amorphous and crystalline SiO2 have been thought to be related to oxygen vacancies for >50years. However, direct proof for oxygen vacancies from experimental 17O hyperfine data has never been provided. In this contribution, we report on the most complete set of spin Hamiltonian parameter matrices g, A(73Ge), P(73Ge), A(17O), and A(29Si) for the Ge analog of the classic E1′ center (denoted GeE1′), determined from single-crystal electron paramagnetic resonance (EPR) analyses of two fast-electron-irradiated, Ge-doped α-quartz samples, including one grown from 17O-enriched water. These experimental data, particularly the three distinct matrices A(17O), not only confirm the GeE1′ center to represent spin trapping on a substitutional Ge atom coordinated to three nonequivalent nearest-neighbor O atoms (i.e., the GeO moiety involving an oxygen vacancy) and two next-nearest-neighbor Si atoms but also permit determination of the spin population on all of these atoms. These EPR data also have been evaluated by periodic density functional theory (DFT) calculations to demonstrate that the widely popular single-oxygen-vacancy V(O)+ model for the GeE1′ center cannot account for A(17O) arising from three nonequivalent nearest-neighbor oxygen atoms. A new tri-vacancy with an Al impurity model V(SiO2)Al, on the other hand, reproduces all the EPR experimental hyperfine matrices A(73Ge), A(17O), and A(29Si) and explains the common associations of the E′ centers with both Al related defects and superoxide/peroxy radicals in quartz and amorphous silica.

Models of triplet self-trapped excitons in SiO2, HfO2, and HfSiO4

We predict by means of density functional theory simulations the structures of self-trapped triplet excitons in HfO${}_{2}$ and HfSiO${}_{4}$, and compare their properties to those of the self-trapped exciton in $\ensuremath{\alpha}$ quartz. The character of the hole and electron localization in excitons strongly depends on the geometrical structure and dielectric properties of these materials. In HfO${}_{2}$, the electron is localized predominantly on one Hf atom while the hole is localized on one or two oxygen atoms at the nearest or next-nearest-neighbor sites, depending on the crystal phase. We predict two exciton configurations in HfSiO${}_{4}$ with the excited electron localized either on a Hf or on a Si atom and the hole localized on the nearest-neighbor oxygen atoms. Excitation and luminescence energies are calculated for all triplet exciton states and compared with the available experimental data.

Structural characterization of titanium-doped Bioglass using isotopic substitution neutron diffraction

Melt quenched silicate glasses containing calcium, phosphorus and alkali metals have the ability to promote bone regeneration and to fuse to living bone. Of these glasses 45S5 Bioglass® is the most widely used being sold in over 35 countries as a bone graft product for medical and dental applications; particulate 45S5 is also incorporated into toothpastes to help remineralize the surface of teeth. Recently it has been suggested that adding titanium dioxide can increase the bioactivity of these materials. This work investigates the structural consequences of incorporating 4 mol% TiO(2) into Bioglass® using isotopic substitution (of the Ti) applied to neutron diffraction and X-ray Absorption Near Edge Structure (XANES). We present the first isotopic substitution data applied to melt quench derived Bioglass or its derivatives. Results show that titanium is on average surrounded by 5.2(1) nearest neighbor oxygen atoms. This implies an upper limit of 40% four-fold coordinated titanium and shows that the network connectivity is reduced from 2.11 to 1.97 for small quantities of titanium. Titanium XANES micro-fluorescence confirms the titanium environment is homogenous on the micron length scale within these glasses. Solid state magic angle spinning (MAS) NMR confirms the network connectivity model proposed. Furthermore, the results show the intermediate range order containing Na-O, Ca-O, O-P-O and O-Si-O correlations are unaffected by the addition of small quantities of TiO(2) into these systems.

Charge distribution and oxygen diffusion in hyperstoichiometric uranium dioxide UO 2+x ( x ⩽ 0.25)

Quantum–mechanical techniques were used to determine the charge distribution of U atoms in UO 2+ x ( x ⩽ 0.25) and to calculate activation-energy barriers to oxygen diffusion. Upon optimization, the reduction in unit-cell volume relative to UO 2, and the shortest 〈U–O〉 and 〈O–O〉 bond-lengths (0.22 and 0.24 nm, respectively) are in good agreement with experimental data. The addition of interstitial oxygen to the unoccupied cubic sites in the UO 2 structure deflects two nearest-neighbor oxygen atoms along the body diagonal of uranium-occupied cubic sites, creating lattice oxygen defects. In (1 × 1 × 2) supercells, the partial oxidation of two U 4+ atoms is observed for every interstitial oxygen added to the structure, consistent with previous quantum–mechanical studies. Results favor the stabilization of two U 5+ over one U 6+ in UO 2+ x . Calculated activation energies (2.06–2.73 eV) and diffusion rates for oxygen in UO 2+ x support the idea that defect clusters likely play an increasingly important role as oxidation proceeds.

O and H diffusion in uraninite: Implications for fluid–uraninite interactions, nuclear waste disposal, and nuclear forensics

Diffusion coefficients for oxygen and hydrogen were determined from a series of natural uraninite–H 2O experiments between 50 and 700 °C. Under hydrous conditions there are two diffusion mechanisms: (1) an initial extremely fast-path diffusion mechanism that overprinted the oxygen isotopic composition of the entire crystals regardless of temperature and (2) a slower volume-diffusive mechanism dominated by defect clusters that displace or eject nearest neighbor oxygen atoms to form two interstitial sites and two partial vacancies, and by vacancy migration. Using the volume diffusion coefficients in the temperature range of 400–600 °C, diffusion coefficients for oxygen can be represented by D = 1.90e −5 exp (−123,382 J/RT) cm 2/s and for temperatures between 100 and 300 °C the diffusion coefficients can be represented by D = 1.95e −10 exp (−62484 J/RT) cm 2/s, where the activation energies for uraninite are 123.4 and 62.5 kJ/mol, respectively. Hydrogen diffusion in uraninite appears to be controlled by similar mechanisms as oxygen. Using the volume diffusion coefficients for temperatures between 50 and 700 °C, diffusion coefficients for hydrogen can be represented by D = 9.28e −6 exp (−156,528 J/RT) cm 2/s for temperatures between 450 and 700 °C and D = 1.39e −14 exp (−34518 J/RT) cm 2/s for temperatures between 50 and 400 °C, where the activation energies for uraninite are 156.5 and 34.5 kJ/mol, respectively. Results from these new experiments have implications for isotopic exchange during natural UO 2–water interactions. The exceptionally low δ 18O values of natural uraninites (i.e. −32‰ to −19.5‰) from unconformity-type uranium deposits in Saskatchewan, in conjunction with theoretical and experimental uraninite–water and UO 3–water fractionation factors, suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low δ 18O values have been interpreted as resulting from the low temperature overprinting of primary uranium mineralization in the presence of relatively modern meteoric fluids having δ 18O values of ca. −18‰, despite petrographic and U–Pb isotope data that indicate limited alteration. Our data show that the anomalously low oxygen isotopic composition of the uraninite from the Athabasca Basin can be due to meteoric water overprinting under reducing conditions, and meteoric water or groundwater can significantly affect the oxygen isotopic composition of spent nuclear fuel in a geologic repository, with minimal change to the chemical composition or texture. Moreover, the rather fast oxygen and hydrogen diffusion coefficients for uraninite, especially at low temperatures, suggest that oxygen and hydrogen diffusion may impart characteristic isotopic signals that can be used to track the route of fissile material.

Local structure of Ba(Ti,Zr)O3perovskite-like solid solutions and its relation to the band-gap behavior

Local structures in BaTi{sub 1-x}Zr{sub x}O{sub 3} solid solutions were analyzed using x-ray absorption fine structure (XAFS) measurements and density-functional theory (DFT) calculations. We demonstrate that for low concentrations of Ti, isolated Ti atoms in the relatively large octahedral sites of the BaZrO{sub 3} lattice acquire centrosymmetric coordination with average Ti-O distances shorter than those in BaTiO{sub 3}. In contrast for higher concentrations of Ti, Ti atoms having one or more Ti as their B-site nearest neighbors undergo polar off-center displacements. Our DFT calculations confirm both effects. These results combined with the previously published data suggest that isolated polarizable ions on the B sites of a relatively expanded host perovskite lattice remain nonpolar by symmetric relaxation of the nearest-neighbor oxygen atoms to yield nearly ideal bond lengths around the dopant species. For neighboring Ti atoms, such symmetric relaxation is impossible, and these atoms are displaced off center. Our XAFS measurements did not detect any significant deviations from a random distribution of Ti and Zr in the present samples except for compositions close to BaTiO{sub 3}. The DFT calculations suggest that the dominant effect of the local displacements on band-gap values for this system is determined by the shortest Ti-O bondsmore » due to strong Ti 3d-O 2p hybridization; however, local displacements have only a secondary effect on the band-gap behavior.« less

Ferromagnetic properties, electronic structures, and formation energies of Zn vacancy monodoping and (Zn vacancy, Li) codoped ZnO by first principles study

Using the first-principles method based on the density functional theory, we investigated the ferromagnetic properties, electronic structures, and formation energies of Zn vacancy monodoping and (Zn vacancy, Li) codoped ZnO. The results indicate that both cases prefer the ferromagnetic ground state. It was found that the Zn vacancy defect brings a spin polarized state in the nearest neighbor oxygen atoms, and the magnetic moments mainly come from the O atoms surrounding the defect centers, which are different from the conventional diluted magnetic semiconductor. In addition, we found that the spin polarized oxygen atoms have a metallic feature in both spin states and the ferromagnetic exchange interaction among oxygen atoms is mediated by Zn 3d state. Furthermore, it was observed that the replacement of one Zn atom in the system of Zn15O16 by one Li atom can generate holes and reduce the formation energy of Zn vacancy, and then stabilizes the zinc vacancy-including system, resulting in a larger magnetic moment.

X-ray photoelectron spectroscopy characterization of fluorite and perovskite phases in Sr1−xBi2+yTa2O9−z films

High-resolution x-ray photoelectron spectroscopy of strontium bismuth tantalate films revealed distinct chemical states of constituent atoms in fluorite and perovskite crystals. Reflecting the number of nearest-neighbor oxygen atoms coordinated with Bi3+ ions, the Bi 4f peak could be deconvoluted into Bi2O3 (sixfold coordination), Bi2O2 (fourfold coordination), and (Bi2O2)2+ (intermediate between sixfold and fourfold coordination) components. We found that amorphous and fluorite phases could be expressed as a mixture of Bi2O3 and Bi2O2, whereas the (Bi2O2)2+ component representing the bismuth oxide layer, being adjacent to the (SrTa2O7)2− block, predominated in the perovskite phase. Similarly, the Sr 3d peak could be deconvoluted into the components of Sr oxide (SrO) and Sr in (SrTa2O7)2−. The volume fraction of the (SrTa2O7)2− component increased at higher annealing temperatures. The Ta 4d peaks, in contrast, located at the same binding energy for all samples, indicated that an energetically stable TaO6 octahedron unit is preferentially created. The binding energies of the O 1s state corresponding to O–Bi, O–Ta, and O–Sr bonds were identified at 531, 530, and 528.9 eV, respectively.

Raman and X-ray absorption spectroscopic studies of hydrothermally altered alkali-borosilicate nuclear waste glass

Abstract Raman spectroscopy and X-ray absorption spectroscopy (XAS) are used to characterize structural changes that took place in hydrothermally altered (Na,K)-alumina-borosilicate glasses with different Na/K ratios, formulated as part of a durability study to investigate the behavior of glasses for nuclear waste storage. The hydrothermal experiments, or vapor hydration tests (VHT), were performed on each glass for 3 and 20 days at 200 °C to accelerate and approximate long-term alteration processes that may occur in a nuclear waste repository. Results found for both glasses and their VHT altered counterparts show little, if any, structural influence from the different starting Na/K ratios. X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and Raman spectroscopy indicate that the altered samples are mostly amorphous with small amounts of analcime-like and leucite-like crystals within 200 μm of the sample surface and contain up to 9.7 wt.% water or OH. The Raman data are nearly identical for the amorphous portions of all altered VHT samples investigated, and indicate that two glass structural changes took place during alteration: one, partial depolymerization of the alumina-borosilicate network, and two, introduction of water or OH. Al and Si XAS data indicate tetrahedral AlO 4 and SiO 4 environments in the original glasses as well as in the altered samples. Small energy shifts of the Si K-edge also show that the altered VHT samples have less polymerized networks than the original glass. Na XAS data indicate expanded Na environments in the VHT samples with longer Na–O distances and more nearest-neighbor oxygen atoms, compared with the original glasses, which may be due to hydrous species introduced into the expanding Na-sites.

Ferromagnetism induced by Zn vacancy defect and lattice distortion in ZnO

Through the full potential linearized augmented plane wave method, we have explored the vacancy defect induced magnetism in wurtize ZnO. It has been found that the Zn vacancy defect brings a spin polarized state in the nearest neighbor oxygen atoms, whereas the oxygen vacancy defect has no influence on the magnetism. However, it is found that the lattice distortion is a crucial factor for the Zn vacancy induced ferromagnetism because the ferromagnetic ground state cannot be achieved if there is no lattice distortion due to Zn vacancy defect. The magnetic moment of oxygen atom in the nearest neighbor from the Zn vacancy site is ranged from 0.10 to 0.19 μB and the spin polarized oxygen atoms have metallic feature in both spin states. These results are quite different from those found in other calculation [Q. Wang et al., Phys. Rev. B 77, 205411 (2008)]. In addition, we have found that the ferromagnetic exchange interaction among oxygen atoms is mediated by Zn 3d state. Along with these, the theoretically calculated x-ray absorption spectroscopy and x-ray magnetic circular dichroism are presented.

Energy Shifts of Si Oxidation States in the System of Si Nanocrystals Embedded in SiO<SUB>2</SUB> Matrix

Energy shifts in the Si 2p levels of the five Si oxidation states Sin+ (n = 0, 1, 2, 3, 4) in the system of Si nanocrystals embedded in SiO2 matrix have been determined. The thermal annealing effect on the energy shifts has been studied. The result suggests that the Si nanocrystals and the SiO2 are thermally stable but the annealing can cause some structural deformations such as changes in the bond lengths and bond angles for the suboxides Si2O and SiO. The energy shifts generally show a linear dependence on the oxidation state n, suggesting that the energy shifts could be mainly determined by the nearest-neighbor oxygen atoms. It is shown that the chemical structures of the system are similar to those of the conventional SiO2/Si system in terms of the energy shifts.

Imaging Water Dissociation on TiO2(110): Evidence for Inequivalent Geminate OH Groups

Identical regions of partially reduced TiO2(110) surfaces with bridge-bonded oxygen vacancy (BBO(V)) concentrations of approximately 10% ML (1 ML = 5.2 x 10(14) cm(-2)) were imaged using scanning tunneling microscopy (STM) before and after dosing H2O at ambient temperature (approximately 300 K). Atomically resolved images confirm that H2O adsorbs dissociatively on the BBO(V) sites, producing two hydroxyl species, one positioned at BBO(V) and denoted OH(V) and the other, denoted OH(B), formed by protonation at either of the two nearest-neighbor bridge-bonded oxygen atoms. Hydrogen hopping along the [001] direction is observed at ambient temperature, with a strong preference for OH(B) (approximately 10x) hydrogen motion. This powerful imbalance demonstrates the inequality of OH(V) and OH(B) and suggests differences in their charge and/or binding configuration.

Hydration of Short-Chain Poly(oxyethylene) in Carbon Tetrachloride: An Infrared Spectroscopic Study

Hydration of short-chain poly(oxyethylene)s, CH(3)(OCH(2)CH(2))(m)OCH(3) (abbreviated as C(1)E(m)()C(1)) (m = 1-3), in carbon tetrachloride has been studied by infrared spectroscopy. The O-H stretching vibrations of water in ternary solutions with H(2)O:C(1)E(m)C(1):CCl(4) mole ratios of 0.000418:0.005:0.995 to 0.000403:0.04:0.96 were analyzed. Two types of hydrogen bonds are formed in the interaction between water and C(1)E(m)C(1) in carbon tetrachloride; one is a monodentate hydrogen bond, in which only one of the O-H bonds of a water molecule participates in hydrogen bonding, and the other is a bidentate hydrogen bond, in which both of the O-H bonds of a water molecule participate in hydrogen bonding by bridging oxygen atoms separated by two or more monomer units on the polymer chain. An important finding is that the bidentate hydrogen-bond bridge is not formed between the nearest-neighbor oxygen atoms. This experimental observation supports the results of previous molecular dynamics simulations. The shortest oligomer of poly(oxyethylene), i.e., CH(3)OCH(2)CH(2)OCH(3) (1,2-dimethoxyethane) with a single monomer unit, is suggested not to be an adequate model for this polymer with respect to hydrogen bonding to water. The hydrogen bonding in a 1:1 C(1)E(m)C(1)-water adduct in carbon tetrachloride represents primitive incipient hydration of poly(oxyethylene). The present results indicate that both monodentate and bidentate hydrogen bonds are important and the latter is destabilized more rapidly than the former with increasing temperature. This dehydration process can be a potential mechanism of the poly(oxyethylene)-water phase separation.

A local structure of low coverage Ni species on the α-Al 2O 3 (0 0 0 1) surface – a polarization dependent EXAFS study

The geometrical structure of a low coverage Ni species on an α-Al 2O 3 (0 0 0 1) single crystal surface has been studied using a polarization-dependent total reflection fluorescence extended X-ray absorption fine structure (PTRF-EXAFS) spectroscopy to reveal the bonding feature between the metal and the oxide surface. The Ni species on the α-Al 2O 3 (0 0 0 1) surface is atomically dispersed with a Ni–O distance at 0.198 ± 0.003 nm. The adsorption site is determined to be a threefold oxygen site on top of a deep Al atom accompanied with a relaxation of the nearest neighbor oxygen atoms.

Identification and study of vacancies in titanium monoxide by means of positron annihilation techniques

Structural vacancies are of interest for an understanding of the crystal structure and the properties of metal oxides. Here we report on the identification and study of structural vacancies in titanium monoxide TiOy with a B1 (NaCl) structure. Structural vacancies were identified by coincident Doppler broadening studies of the positron–electron annihilation radiation. The analysis shows that the nearest-neighbor atoms of vacancies are oxygen atoms. Taking into account the B1 structure of titanium monoxide this means that vacancies are located on the titanium sublattice. Positron trapping in titanium vacancies allows the study of the electronic structure of these vacancies by positron lifetime spectroscopy which shows that the valence electron density in the titanium vacancy increases with decreasing oxygen content y in TiOy. The increase of the valence electron density is accompanied by a decrease of the mean number of nearest-neighbor oxygen atoms in the vicinity of the titanium vacancy from 6 to 5 atoms.

Low-temperature Ar/N2 remote plasma nitridation of SiO2 thin films

Low-temperature nitridation of SiO2 thin films by Ar/N2 remote plasma processing was investigated using on-line Auger electron spectroscopy, angle-resolved x-ray photoelectron spectroscopy (ARXPS), and optical emission spectroscopy (OES). Nitridation experiments were performed at 300 °C using 30 W Ar/N2 remote plasmas at 0.1 and 0.3 Torr. Ar/N2 remote plasma exposure of 5 nm SiO2 films for 30 min results in nitrogen incorporation throughout the films, independent of process pressure and plasma reactor configuration (i.e., upstream versus downstream N2 injection). ARXPS indicates a N–Si3 local bonding configuration with second nearest neighbor oxygen atoms. Ar/N2 remote plasma exposure at 0.1 Torr results in higher nitrogen concentrations (8–10 at. %). Reactor configuration has a negligible effect at 0.1 Torr; conversely, downstream N2 injection results in higher nitrogen concentrations (5–6 at. %) than upstream injection (3–4 at. %) at 0.3 Torr. OES indicates that the Ar/N2 remote plasmas contain N2 triplet excited states and ground-state N atoms. The Ar emission intensities and the saturation N concentrations in the resultant films follow similar trends with processing pressure and reactor configuration; the N2 first positive emission intensities run counter to these trends. We infer that low-temperature SiO2 nitridation by Ar/N2 remote plasmas is a two-step process: O removal by Ar+ ion bombardment and N insertion by plasma-generated active N species. Moreover, the first step appears to be rate limiting under the conditions employed in this study. Annealing the oxynitride films in N2 at 900 °C decreases the N concentration and results in a more uniform nitrogen distribution.

An X-ray absorption spectroscopy study of neptunium(V) reactions with Mackinawite (FeS).

Neptunium is a transuranium element, produced in tonne quantities in nuclear reactors. Because it has access to a range of oxidation states, neptunium may undergo redox transformations in the environment and these can have far-reaching effects on its environmental mobility. Here, the reaction of NpO2+ (the soluble and thermodynamically stable neptunium species in oxic systems) with microcrystalline mackinawite is studied. Uptake of neptunium from solution is relatively low (approximately 10% of the total initially present in solution) and independent of initial solution concentration over the range 0.27-2.74 mM and of equilibration time. X-ray absorption spectroscopy (XAS) of the solid sulfide samples indicates nearest neighbor oxygen atoms at distances around 2.25-2.26 A, sulfur atoms at around 2.61-2.64 A, and two more distant shells fitted with iron, at 3.91-3.95 A and 4.15-4.16 A. These observations suggest that on interaction with the sulfide surface reduction of Np(V) to Np(IV) occurs, accompanied by loss of axial oxygen atoms. Neptunium coordinates directly to surface sulfide atoms, in contrast to the behavior previously observed for uranium under similar conditions. These results demonstrate the importance and variability of the speciation of redox sensitive actinides under anoxic conditions.

Effect of Temperature on Local Structure in Poly(ethylene oxide)−Zinc Bromide Salt Complexes

Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize the (PEO)n[ZnBr2(1-x)(LiBr)x] system. Values of n and x were chosen so that the average number of nearest-neighbor bromine atoms varied from 2.25 to 3 as determined by Raman spectroscopy. This study determined that the number of nearest-neighbor oxygen atoms was 3, 1, and 0 for zinc with 2, 3, and 4 nearest-neighbor bromine atoms, respectively. Some ambiguity is present in the first case, with four nearest-neighbor oxygen atoms being almost impossible and two being possible but not likely. The mean-squared displacement of the zinc−bromine bond increased almost linearly with absolute temperature, similar to the behavior observed for other covalent or ionic bonds. Interestingly, the strength of the Zn−Br bond, as determined by the change of the mean-squared displacement with temperature, was the same for all samples independent of the degree and nature of coordination. For some samples, there was a clear rise in the mean-...

Structures of pure and Ca-segregated MgO (001) surfaces

The structures of pure and Ca-segregated MgO (001) surfaces have been studied using first-principles density-functional theory. The relaxation and rumpling for the pure surface are found to be 0.48% and 1.62%, respectively. Ca segregation significantly modifies the surface structure. The surface-segregated Ca atoms protrude outwards owing to the size mismatch between Ca and Mg. Consequently, their nearest neighbor oxygen atoms are pulled up. The value of the protrusion of Ca atoms is strongly dependent on the Ca coverage of the surface.

Vibrations of copper atoms in Pr2CuO4

The method of isotopic contrast in combination with inelastic neutron scattering has been used to investigate the vibrational spectrum of the copper atoms in Pr2CuO4. It is shown that the energy positions of the features of the vibrational spectrum of copper in Pr2CuO4 and CuO (which also has a planar oxygen coordination of copper) coincide. We conclude that the dynamical behavior of the copper atoms is formed mainly by their interaction with the nearest-neighbor oxygen atoms.