Vacancy-impurity Complexes Research Articles

First-principles study of vacancy-impurity (C, N, O) interactions in vanadium

Abstract Interstitial impurities of C, N, and O typically exist in V and its alloys, which have significant effects on the structure and properties of the materials due to their strong interaction with the vacancies. In this study, the structure and energy properties of vacancy-impurity complexes in V are comprehensively investigated by first principles calculations. The effects of impurity concentration and temperature on the concentration distribution of the complexes were studied using statistical methods. The results showed that the Vac n X 2 complexes exhibit a high sequential binding energy. There is an evident mutual attraction between the vacancy and the O atom, and it has a higher affinity than the C or N atom. The simple defect complexes of Vac 1 X 1–2 have always been the most abundant type of complex defect in V. The trapping effect of vacancy on C, N, and O atom is very significant at low temperatures rather than at high temperatures. This study would broaden the understanding of vacancy-impurity interactions in vanadium and provide new insights into the influence of C, N, and O on vacancy defect concentrations.

Crystal Growth and Spectroscopy of Yb2+-Doped CsI Single Crystal

The single crystals of CsI-Yb2+ were grown, and their spectroscopic studies were conducted. The observed luminescence in CsI-Yb2+ is due to 5d–4f transitions in Yb2+ ions. Using time-resolved spectroscopy, spin-allowed and spin-forbidden radiative transitions of ytterbium ions at room temperature were found. The excitation spectra of Yb2+ luminescence bands were obtained in the range of 3–45 eV. The mechanism of charge compensation of Yb2+ ions in a CsI crystal was also studied, the spectrum of the thermally stimulated depolarization current was measured, and the activation energies of the two observed peaks were calculated. These peaks belong to impurity–vacancy complexes in two different positions. The charge compensation of Yb2+ occurs via cation vacancies in the nearest-neighbor and next-nearest-neighbor positions.The Yb2+ ions are promising dopants for CsI scintillators and X-ray phosphors in combination with SiPM photodetectors.

Separation in Fe–Ni and Fe–Ni–P alloys under electron irradiation and isochronous annealing

The annealing of radiation defects in the Fe–34.7at.%Ni and Fe–34.6at.%Ni–0.1at.%P alloys irradiated with 5 MeV electrons is studied by the method of residual electrical resistivity. It is shown that, during annealing after irradiation at 80 K, vacancies become mobile in the region of 220 K with a migration energy of about 0.6 eV. In this case, vacancy-impurity complexes (clusters) are formed, and in the H36 alloy vacancy clusters are formed. Under irradiation at room temperatures, vacancy defects accumulate in the form of vacancy clusters. The dissociation of these clusters at 350–550 K results in the appearance of freely migrating vacancies and enhanced self-diffusion. This leads to radiation-accelerated ordering processes in the Fe–Ni and Fe–Ni–P alloys. At temperatures around 800 K, homogenization of the solid solution occurs. The details and stages of the dissociation of vacancy-impurity complexes are discussed.

Local structure of an oxygen impurity and fluorine vacancy complex in LiYF4

Crystal-growth defects and impurities are among the key factors limiting the performance of fluorine-based optical materials. This study reports the local structure of an oxygen impurity centre in LiYF4 single-crystal. Experimental electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy investigations were combined with first principle theoretical modelling to elucidate the defect structure. It was determined that, after irradiation with X-rays, an electron spin S=1/2 paramagnetic centre is generated. It is stable at room temperature and anneals above 150°C. The alignment of the g-tensor and magnitudes of hyperfine interactions suggest that the centre is an oxygen impurity and fluorine vacancy complex. Highly anisotropic hyperfine interaction with three fluorine atoms within 3 Å of the oxygen defect is observed. The results provide novel insights into oxygen ion incorporation in LiYF4 and are crucial to understanding and minimising radiation-induced degradation of the material.

Механизм распухания облучаемого оксидного ядерного топлива. Часть II. Распухание при скачках мощности

Analysis of swelling of large grain fuel with different additives and fission gas release from such fuel during power ramps is carried out. It is shown that the fuel swelling is phase transition of 1st kind in the supersaturated solid solution of vacancy-impurity complexes (Xe with vacancies) in uranium dioxide. Accelerated fuel swelling during power ramps is a case of thermo-kinetic effect occurred in supersaturated systems under heating/cooling.

Механизм распухания облучаемого оксидного ядерного топлива. Часть I. Режим штатного облучения

The fuel swelling during normal operation is considered within the framework of 1st kind phase transition theory. It is shown that the phase transition occurs in the supersaturated sold solution of vacancy-impurity complexes – Xe with vacancies in uranium dioxide. The analysis of fuel swelling and fission gas release from fuel doped with different additives during normal operation was carried out.

Influence of confinement and surface functionalization effects on fluorescent properties of the impurity–vacancy complexes in ultrasmall nanodiamonds

We have done a thorough ab-initio research of vacancy–impurity complexes (including nitrogen, silicon and germanium) in 0.65 nm thick diamond nanocrystal and compared the results with those obtained for the complexes, located in diamond nanofilms of the same thickness and bulk crystals. While we consider only hydrogen-terminated and bare surfaces of nanoparticles and nanofilms , we are able to investigate the influence of confinement effects onto formation energy and electron structure of the impurity levels in the vicinity of band gap. It was found, that formation energy of these centers in nanoobjects (nanofilms and nanoparticles) is 2 eV less than that of bulk crystals. This implies preferable formation of these centers during growth close to the surface or in ultrasmall nanodiamonds. Confinement effects also significantly affect the band structure of nanoobjects. Beside the well-known effect of the band gap widening, they also lead to “popping up” of filled impurity levels over the valence band maximum. For negatively charged NV− center, located in nanodiamond, this results in interference of the upper, partially filled impurity level with the surface states, bringing about the splitting of this impurity level. We predict, that this effect would manifest itself in intermittent luminescence of NV− doped ultrasmall nanodiamonds with size below 5 nm.

Effect of purity on the vacancy defects induced in self–irradiated tungsten: A combination of PAS and TEM

Vacancy defects in tungsten induced by self-ion irradiation were characterized by positron annihilation spectroscopy (PAS) and transmission electron microscopy (TEM). Taking advantage of their complementarity, defects ranging from single vacancies up to vacancy clusters were detected, and the effects of sample purity (99.95 wt.% and 99.9999 wt.%) on their formation and evolution under low damage dose (0.01 to 0.02 dpa) irradiation with 20 and 1.2 MeV-W ions were studied at room temperature (RT), 500 and 700 °C respectively. When irradiation was performed at RT, PAS probed mainly single vacancies. At high irradiation temperatures, larger vacancy clusters were detected by both techniques. Both PAS and TEM revealed larger vacancy clusters in the purest samples after irradiation at 500 and 700 °C. A significant difference in the evolution of vacancy-type defects related to sample purity was observed. In view of the properties (migration, trapping) of some light-element impurities (LEs), present at high concentrations, determined by first-principles calculation, it is reasonable to assign the effect of purity during the irradiation response to the formation of vacancy-impurity complexes, which is critical for the understanding of the behavior of tungsten in the future fusion reactor.

Recovery, recrystallization and diffusion in cold-rolled Ni

Abstract Recovery and recrystallization processes in cold-rolled Ni are investigated. While recrystallization and grain growth at temperatures above 700 K lead to a significant (threefold) decrease in micro-hardness, recovery at 600 K is shown to cause a measurable micro-hardness increase. This increase in micro-hardness is confirmed by atomic force microscopy observations of Vickers indents, and it is correlated with the vacancy-annihilation peak of the calorimetric signal. Diffusion of impurity–vacancy complexes to the dislocation cores and the resulting pinning of dislocations are surmised to contribute to the measured micro-hardness increase. Self-diffusion along interfaces in cold-rolled and partially recrystallized Ni samples is measured employing the radiotracer serial sectioning technique. Diffusion rates similar to those of general high-angle grain boundaries in well-annealed coarse-grained Ni polycrystals are observed. The results imply that the diffusion rate along the stationary recrystallization front, i. e. the interfaces separating the recrystallized and the as-deformed regions of the material, is similar to that along a general high-angle grain boundary.

Locally symmetric oxygen vacancy around Cd impurities in CeO2

A detailed investigation of the electronic structure in the neighborhood of Cd impurities in ${\mathrm{CeO}}_{2}$ has been performed by ab initio calculations to elucidate the interplay between the oxygen vacancies and electric quadrupole interactions. The quadrupole frequency related to the major component of the electric-field gradient (EFG) at impurities sites from its neighboring charge density as well as its symmetry were calculated by simulating oxygen vacancies at oxygen nearest neighbor of Cd. Results show a very good agreement with experimental hyperfine interactions measurements at the $^{111}\mathrm{Cd}$ nucleus replacing Ce at ${\mathrm{CeO}}_{2}$. A systematic mapping of oxygen vacancies in ${\mathrm{CeO}}_{2}$ supercells was proposed within the framework of density-functional theory using the wien2k code focusing on the electronic distribution in the vicinity of Cd impurities. Results show that the calculated values of EFG crucially depend on impurity-vacancy complex position and the striking axial symmetry observed when an oxygen monovancy is at the nearest neighborhood of Cd is explained by a rearrangement of its $p$ orbitals.

Assessment of positrons for defect studies in CeO2 materials

Disposal of spent nuclear fuel poses significant challenges, as the UO2 and fission products are under constant irradiation and must be safely stored for millennia. CeO2 is a non-radioactive analog for UO2 for studying microstructure and its evolution. Many techniques have been applied to uranium and cerium oxides to investigate point defects. Positron annihilation spectroscopies (PAS) are sensitive to neutral and negatively charged vacancy-like point defects and impurity vacancy complexes. PAS has been applied previously to UO2+x to investigate nuclear fuels, but virtually no PAS work exists on CeO2. Here, the basics of positron annihilation spectroscopy is reviewed, and preliminary work on undoped and doped CeO2 is shown and compared to the literature results from UO2. To simulate fission product incorporation in spent nuclear fuels, CeO2 samples were doped at different concentration with yttrium. Select samples were irradiated with heavy ions at different doses. Doping and irradiation are shown to give rise to different defect characteristics.

The effect of vacancy-impurity complexes in silicon on the current–voltage characteristics of p–n junctions

The methods for analyzing the current–voltage characteristics of p–n junctions at forward and reverse bias with the calculation of the parameters of recombination centers before and after irradiation with gamma quanta were developed in this work. These methods are simple, convenient, and allow one to determine the parameters of deep levels at one temperature, which makes it possible to use them as express diagnostic techniques, taking measurements immediately on the semiconductor plates at the completion of the main technological processes. It was shown that after irradiation the composition of the centers in the depletion region of the p–n junction changes, the recombination processes and the lifetime begin to determine the divacancy complexes of silicon with oxygen (V2O). It was found that the generation of charge carriers in a strong field of the depletion region of the p–n junction occurs under the influence of the electron–phonon interaction, the parameters of this interaction are determined, and the configuration-coordinate diagrams of recombination centers are constructed.

Short positron lifetime at vacancies observed in electron-irradiated tungsten: Experiments and first-principles calculations

A positron lifetime component of ~170 ps has been reported for irradiated tungsten (W) in some studies. This value is considerably shorter than that calculated for isolated monovacancies (~200 ps). In this study, positron annihilation lifetime spectroscopy was used to investigate the recovery behavior of the defects with a short positron lifetime of ~170 ps in electron-irradiated W. The binding energies and positron lifetimes of vacancy-impurity complexes decorated with impurity atoms were calculated. A positron lifetime of 169 ps, which was similar to the experimentally observed lifetime of 171 ± 1 ps, was calculated for defect complexes comprising a monovacancy decorated with two hydrogen atoms (V-2H complexes). In addition, a value of 1.42 eV was calculated for the dissociation energy of a hydrogen atom from the V-H and V-2H complexes, and the defects with a positron lifetime of ~170 ps were experimentally observed to migrate at 623 K. These were consistent with previously reported dissociation energy and desorption temperature of deuterium from vacancies in W studied by thermal desorption spectroscopy. These results suggested that the vacancies observed in the electron-irradiated W were hydrogen-decorated V-nH complexes.

Luminescence of F-Type Defects and Their Thermal Stability in Sapphire Irradiated by Pulsed Ion Beams

We studied the luminescence and the thermal stability of defects formed in α-Al2O3 single crystals after pulsed treatment with a beam of C+/H+ ions with an energy of 300 keV and a pulse duration of ~80 ns. By measuring optical absorption, photoluminescence, and pulsed cathodoluminescence, it is found that this type of exposure leads to intense generation of both single F and F+ centers and more complex defects (F2 aggregate centers or vacancy–impurity complexes) in α-Al2O3. The thermal stability of F-type defects formed in α-Al2O3 upon exposure to a pulsed ion beam is comparable to the stability of radiation-induced defects in neutron-irradiated samples.

Люминесценция дефектов F-типа и их термическая стабильность в сапфире, облученном импульсными ионными пучками

Luminescence and thermal stability of defects formed in alpha-Al2O3 single crystals under pulsed ion beam treatment (C+/H+ ions with an energy 300 keV, pulse duration 80 ns) were investigated. This type of irradiation leads to the intensive generation of both single F- and F+-centers and more complex defects (F2-type aggregate centers or vacancy-impurity complexes) in alpha-Al2O3. It was confirmed by the results of optical absorption, photoluminescence, and pulsed cathodoluminescence measurements. The thermal stability of F-type defects formed in alpha-Al2O3 under the pulsed ion beam treatment is comparable to the stability of radiation-induced defects in neutron-irradiated samples.

Effect of lonsdaleite on the optical properties of impact diamonds

The special features of impact diamonds are the orientation of the nanosized grains relative to each other, the presence of hexagonal diamond (lonsdaleite, L) in a large part of the samples and the increased wear resistance. Using Raman spectroscopy and XRD, two groups of translucent samples of Popigai impact diamonds (PIDs) were selected: with and without lonsdaleite and the effect of lonsdaleite on the optical properties of the samples was studied. In all L-containing PIDs there is a strong absorption band of about 1230 cm-1 in the one-phonon region, in the mid-IR. The absorption edge is blurred and described by the Urbach rule. The estimated value of Eg ~4 eV for L is consistent with the first principles calculations. Impurity nitrogen is found only in L-free PIDs: There are signals from nitrogen-vacancy complexes in the photoluminescence (PL) spectra. Variations in the number of nitrogen atoms (N = 1 to 4) in the structure of these centers indicate significant variations in the parameters of PID annealing. L-containing PIDs are characterized by large strains in the lattice and, as a consequence, there are problems with the defect diffusion. The narrow lines in PL spectra, uncommon for diamond, can be the result of several orders of magnitude higher concentrations of impurities in PIDs formed during the solid-phase transition. The broadened peaks of 180, 278 and 383 K are distinguishable in the curves of thermostimulated luminescence (TSL) for L-free PIDs, but in the presence of L the TSL glow becomes continuous as in natural IaA-type diamonds with platelets. In general, lonsdaleite deteriorates the optical properties of impact diamonds and makes it difficult to create certain types of impurity-vacancy complexes for different applications.

Ab initio Calculation of Impurity–Vacancy Complexes in Diamond at High Pressure

Optical centers in diamond are possible candidates for single-photon emitters for applications in quantum communication, biology, and medicine. The study of the pressure dependence of zero-phonon lines of these centers can provide deeper insight into the electronic and structural properties of optical centers. These studies can also be useful for fine tuning the photon emission of optical centers in diamond by applying mechanical stresses. The results of ab initio calculations obtained in the study show that the pressure dependence of the position of zero-phonon lines is attributed to the electron density redistribution, whereas the effects associated with an increase in the binding energy under pressure turn out to be an order of magnitude weaker.

Cobalt and Iron Ions in MgO Nanocrystals: Should They Stay or Should They Go

Identification and manipulation of transition-metal ion impurities in oxide nanoparticles require an in-depth understanding of their stability, segregation behavior, and, at the same time, knowledge about their surface reactivity. Powders of magnesium oxide nanoparticles with admixtures of iron or cobalt ions, as two next neighbors in the periodic table, were synthesized in the gas phase via injection of metal–organic precursors into the magnesium combustion flame followed by temperature quenching of resulting nanocrystals in argon. In these model systems of cubic nanocrystals, we explored the distinct stability of these impurities in great detail. While Co2+ ions keep their divalent valence state and substitute the host ions in the cationic sublattice, Fe3+ ions emerge due to the energy gain provided by charge compensation and impurity–vacancy complex formation. The very different behavior of Co and Fe ions in the MgO host lattice, their changes in the local environment, and the different trends in segre...

Effect of short-pulsed ion irradiation on the optical and electrical properties of pyrolytic boron nitride

The effect of irradiation by carbon ions in the mode of short-pulsed ion implantation on the optical and electrical characteristics of graphite-like pyrolytic boron nitride was studied. The characteristics of the states of growth defects localized in the band gap change more significantly with an increase in the energy density of the ion beam, which is accompanied by the ordering of the electronic structure of the compound due to radiation – thermal annealing. Absorption spectra indicate the predominant effect of static disorder due to defects. Irradiation forms a defective semiconductor material with a maximum band gap of 3.3–3.45 eV, a current value of 2.65–2.83 eV for direct transitions and 1.1–1.8 eV for indirect transitions, and an absorption edge due to exponentially distributed states of 1.3–2.6 eV and 2.6–3.3 eV defects of different nature. Anion vacancies, their clusters and impurity-vacancy complexes make the main contribution to the material properties. Irradiation forms low – conducting dielectric layers with surface conductivity of mixed n– and p–type on the surface of a pyrolytic boron nitride. The conduction mechanism is determined by the activation exchange of charge carriers between the allowed bands and donor and acceptor levels due to radiation and growth defects. After irradiation, the Fermi level remains localized near the position characteristic of the pyrolytic boron nitride near the middle of the band gap.

Accumulation of Defects in Austenitic Stainless Steels with Phosphorus and Titanium Additions upon Electron Irradiation at 573 К Investigated Using Positron Annihilation Spectroscopy

Using the positron annihilation spectroscopy method, the accumulation of vacancy-type defects in steels Kh16N15М3 and Kh16N15М3T1 alloyed with phosphorus has been studied at early stages of irradiation at a temperature of 573 К. The obtained data have showed that vacancies in the steels interact with phosphorus atoms at this temperature with the formation of immobile and low-mobile vacancy—impurity complexes, which results in the accumulation of vacancy-type defects. In the steel Kh16N15М3Т1 under irradiation, the formation of nanosized particles of intermetallic Ni3Ti precipitates occurs, which intensify recombination of point defects and reduce their accumulation as compared to the steel Kh16Н15М3. As this occurs, the presence of phosphorus in the steel Х16Н15М3Т1 likewise leads to the intensification of the accumulation of vacancy defects.