Interstitial Pairs Research Articles

Ionic transport of alkali in borosilicate glass. Role of alkali nature on glass structure and on ionic conductivity at the glassy state

Ionic transport of alkali was investigated in the SiO2–B2O3–R2O glass system (with R=Li, Na, K or Cs) by using impedance spectroscopy measurements at the glassy state. Transport mechanisms are also understood on the basis of glass structural analysis, such as X-ray total scattering and Raman spectroscopy, which helped to describe the alkali distribution and the network structure of borosilicate glasses.It was shown that an alkali size increase induces a glass network expansion, with weaker binding forces between alkali and non-bridging oxygens (NBO) and a change in network polymerization, with a Q3 unit increase and a “quasi-absence” of borosilicate unit in the case of lithium glass. The glass structure evolution with alkali size leads to a change in its macroscopic properties, especially glass transition temperature and electrical conductivity.The alkali conduction mechanism is described by an interstitial pair migration based on Frenkel defects in ionic crystals, which is thermally activated. Based on electrical conductivity data, activation energy of conductivity and alkali jump distances were calculated. The latter were linked to the values of bond lengths as determined by X-ray total scattering experiments, which pointed out a likely heterogeneous distribution of alkali in the borosilicate glasses studied.

Palladium-defect complexes in diamond and silicon carbide

Time Differential Perturbed Angular Correlations (TDPAC) studies, supported by Density Functional Theory (DFT) modelling, have shown that palladium atoms in silicon and germanium pair with vacancies. Building on these results, here we present DFT predictions and some tentative TDPAC results on palladium-defect complexes and site locations of palladium impurities in diamond and silicon carbide. For both diamond and silicon carbide, the DFT calculations predict that a split-vacancy V-PdBI-V complex is favoured, with the palladium atom on a bond-centred interstitial site having a nearest-neighbour semi-vacancy on either side. Consistent with experimental results, this configuration is also assigned to palladium complexes in silicon and germanium. For silicon carbide, the DFT modelling predicts furthermore that a palladium atom in replacing a carbon atom moves to a bond-centred interstitial site and pairs with a silicon vacancy to form a complex that is more stable than that of a palladium atom which replaces a silicon atom and then moves to a bond-centred interstitial site pairings with a carbon vacancy. These two competing alternatives differ by 8.94 eV. The favourable pairing with a silicon vacancy is also supported independently by TRIM Monte Carlo calculations, which predict that more silicon vacancies than carbon vacancies are created during heavy ion. implantation.

Identification of triangular-shaped defects often appeared in hard-sphere crystals grown on a square pattern under gravity by Monte Carlo simulations

In this paper, we have successfully identified the triangular-shaped defect structures with stacking fault tetrahedra. These structure often appeared in hard-sphere (HS) crystals grown on a square pattern under gravity. We have, so far, performed Monte Carlo simulations of the HS crystals under gravity. Single stacking faults as observed previously in the HS crystals grown on a flat wall were not seen in the case of square template. Instead, defect structures with triangular appearance in xz- and yz- projections were appreciable. We have identified them by looking layer by layer. Those structures are surrounded by stacking faults along face-centered cubic (fcc) {111}. Also, we see isolated vacancies and vacancy–interstitial pairs, and we have found octahedral structures surrounded by stacking faults along fcc {111}.

Point defects in garnet-type solid electrolyte (c-Li7La3Zr2O12) for Li-ion batteries

Using ab-initio density-functional theory (DFT) methods, the atomic structure and electronic properties of one of the most promising family of solid electrolytes for Li-ion battery applications, lanthanum oxides with a garnet-type structure (c-Li7La3Zr2O12) are studied. The Li-ion (Li+) defects including Li/Li+ vacancies, interstitials, and vacancy–interstitial pair defect formation energy within the Li7La3Zr2O12 supercell are systematically investigated. This study is essential to understand the defect chemistry and the Li+ conductivity mechanisms. Our results indicate that the Li+ vacancy defects are thermodynamically more favorable than interstitial Li+ defects. This work will therefore be helpful to elucidate the atomic level mechanisms of Li defect formation in order to improve the ionic conductivity for future Li-ion battery applications.

Structural modification of single-walled carbon nanotube induced by X-ray irradiation and subsequent annealing studied by Raman scattering spectroscopy in radial breathing mode

The formation of X-ray-induced defects changes the spectral shape of the radial breathing mode (RBM) and defect-induced mode (D band) in the Raman spectra of single-walled carbon nanotubes (SWNTs). X-ray-induced defects have been found to be annealed by thermal treatment, indicating that they are Frenkel pairs (vacancy and interstitial pairs). We found that the spectral shape of RBM is not entirely recovered after post-irradiation annealing. The temperatures for the complete annealing of X-ray-induced defects were within the range of 200–600 °C depending on the tube geometry. From these results, we suggest that the stability of X-ray-induced defects depends on the tube geometry and that the combination of X-ray irradiation and post-irradiation annealing causes a chirality change in SWNTs.

An energetic and kinetic perspective of the grain-boundary role in healing radiation damage in tungsten

Radiation-induced damage in tungsten (W) and W alloys has been considered as one of the most important issues in fusion research, because radiation-produced defects not only degrade the mechanical property but also change the behaviours of H and He in W significantly, such as the retention of H. Nano-structured W has been developed to reduce accumulation of defects within grains and further mitigate radiation-induced damage. However, the fundamental role of a grain boundary (GB) in healing radiation damage in W is not yet well understood. Using molecular dynamics and statics, we evaluate energetically and kinetically the role of a GB in defect evolution (vacancy and interstitial segregation and their annihilation) near the GB in W, by calculating the vacancy (interstitial) formation energy, segregation energy, diffusion barrier, vacancy–interstitial annihilation barrier near the GB and the corresponding influence range of the GB. We find that, as reported and expected, interstitials are preferentially trapped into GBs over vacancies during irradiation, with vacancies dominant near the GB and interstitials highly localized at the GB. On the one hand, the GB serves as a sink both for vacancies and interstitials near itself by reducing their formation energy and diffusion barrier. The formation energy of the vacancy decreases only by ∼0.86 eV, but 7.5 eV is reduced for the formation energy of the interstitial in the GB core, indicating that the sink is unexpectedly stronger for interstitials than vacancies. The average barrier of vacancy diffusion is 0.98 eV much less than 1.8 eV in the bulk; the interstitial migrates into the GB via a barrier-free process. On the other hand, the GB acts as a catalyst for the vacancy–interstitial recombination at the GB by lowering the annihilation barrier. The annihilation with the average barrier as low as 0.31 eV works even at a low temperature of 121 K; besides, the annihilation of a close vacancy–interstitial pair is spontaneous. Meanwhile, the annihilation mechanism near the GB is modified due to the exceptionally large reduction in the interstitial formation energy. The influence range of the GB is small (1–1.5 nm), leading to a small volume fraction of the GB region working as the sink and the catalyst. This indicates that GBs in fine W grains may play a limited role in improving radiation performance.

First-principles study of helium trapping in cementite Fe3C

We report a first-principles density functional theory study of helium distribution in cementite Fe3C. The solution energy of interstitial He is similar to that in bcc Fe; by contrast, the substitutional He (replacing Fe) is remarkably (0.74eV) more stable than in the latter, due to the easiness of Fe vacancy formation in Fe3C. Therefore, He is predicted to be significantly more soluble in cementite than in Fe matrix. We find the binding potencies of both a substitutional–interstitial He pair (0.21eV) and a substitutional–substitutional He pair (0.22eV) are noticeably weaker in cementite than in bcc Fe, indicating a less powerful self-trapping. As a consequence, small size cementite in ferritic steels might serve as scattered trapping centers for He, mitigate helium bubble growth, and make the steel more swelling resistant while under neutron irradiation, just as dispersed oxide particles do.

Exciton Dynamics of CdS Thin Films Produced by Chemical Bath Deposition and DC Pulse Sputtering

Exciton dynamics of CdS films have been investigated using ultrafast laser spectroscopy with an emphasis on understanding defect-related recombination. Two types of CdS films were deposited on glass substrates via direct current pulse sputtering (DCPS) and chemical bath deposition (CBD) techniques. The films displayed distinct morphological, optical, and structural properties. Their exciton and charge carrier dynamics within the first 1 ns following photoexcitation were characterized by femotosecond pump probe spectroscopy. A singular value decomposition (SVD) global fitting technique was employed to extract the lifetime and wavelength dependence of transient species. The excited electrons of the DCPS sample decays through 1.8, 8, 65, and 450 ps time constants which were attributed to donor level electron trapping, valence band (VB) → conduction band (CB) recombination, shallow donor recombination, and deep donor recombination, respectively. The CBD sample shows time constants of 6, 65, and 450 ps which were attributed to CB → VB recombination, sulfur vacancy (VS) recombination, and VS → oxygen interstitial (Oi) donor-acceptor pair (DAP) recombination, respectively. It was found that the DCPS deposition technique produces films with lower defect density and improved carrier dynamics, which are important for high performance solar cell applications.

Defect properties of ZnO nanowires revealed from an optically detected magnetic resonance study

Optically detected magnetic resonance (ODMR) complemented by photoluminescence measurements is used to evaluate optical and defect properties of ZnO nanowires (NWs) grown by rapid thermal chemical vapor deposition. By monitoring visible emissions, several grown-in defects are revealed and attributed to Zn vacancies, shallow (but not effective mass) donor and exchange-coupled pairs of Zn vacancies and Zn interstitials. It is also found that the intensity of the donor-related ODMR signals is substantially lower in the NWs compared with that in bulk ZnO. This may indicate that formation of native donors is suppressed in NWs, which is beneficial for achieving p-type conductivity.

Interaction of n-type dopants with oxygen in silicon and germanium

Density functional theory calculations are employed to gain a fundamental insight on the interaction of n-type dopants such as phosphorous and arsenic with oxygen interstitials and A-centers (vacancy-oxygen interstitial pairs) in silicon and germanium. We propose the formation of the phosphorous-vacancy-oxygen interstitial and arsenic-vacancy-oxygen interstitial cluster in both silicon and germanium.

Poisoning of magnetism in silicon doped with Re, caused by a charge transfer from interstitials to substitutionals, by means of the self-interaction corrected density-functional approach

The self-interaction corrected density-functional calculations are performed for Re impurities and their pairs in silicon. Rhenium ions form in the host crystal not very tight pairs, with impurities separated by one Si atom or by a distance close to two silicon bonds. Comparison of formation energies for various pairs of substitutionals, interstitials, and mixed-site impurities favours the last type. Electron transfer from the interstitial into the substitutional impurity makes the both Re sites nonmagnetic, but the p-type and the n-type co-doping revives magnetism again, the latter more efficiently.

Lifetime of gap discrete breathers in diatomic crystals at thermal equilibrium

Crystals having a gap in a phonon spectrum can support so-called gap discrete breathers (DBs), i.e., nonlinear localized vibrational modes existing in perfect crystals and having frequencies within the gap. In our recent work, for a two-dimensional crystal with stoichiometry A3B, we have demonstrated that if the atomic weight of the components is related as MA MB , then there is a wide gap in the phonon spectrum and the gap DBs can be easily excited. The situation is opposite if the above condition is not satisfied. In the present work the gap DBs are studied at thermal equilibrium, whereas in the previous study the temperature was zero. We demonstrate that in the crystal supporting DBs, in contrast to the opposite case, the lifetime of high-energy light atoms grows with temperature. The possible role of gap DBs in the thermally activated formation of vacancy and interstitial atom pairs (Frenkel pairs) was studied. We have measured the time needed for the appearance of a Frenkel pair as a function of temperature for the crystals supporting and not supporting DBs. A minor difference in waiting time of Frenkel pair nucleation was found in these two cases. We argue that, for the model and its parameters considered in the present work, the main mechanism of thermally activated Frenkel pair formation is the cooperative motion of atoms rather than the excitation of DBs.

Elastic wave propagation in a solid layer with laser-induced point defects

The results of the studies of the propagation of plane harmonic waves and vibrations in an isotropic, elastic solid layer with atomic point defects (interstitial atoms, vacancies, and electron-hole pairs) generated by a pulsed laser beam are presented. The study is based on coupled evolution equations for the elastic displacement of the medium and atomic defect density fields. The defect dynamics are governed by the strain-stimulated generation, diffusion, and annihilation processes. The frequency equations corresponding to the symmetric and antisymmetric elastic-concentration modes of vibration of the layer are obtained. Some limiting cases and special cases of the frequency equations are considered and a procedure for determining the phase velocity and the attenuation (or amplification) constants is discussed.

A-centers and isovalent impurities in germanium: Density functional theory calculations

Abstract In the present study density functional theory calculations have been used to calculate the binding energies of clusters formed between lattice vacancies, oxygen and isovalent atoms in germanium. In particular we concentrated on the prediction of binding energies of A-centers or oxygen interstitials that are at nearest and next nearest neighbor sites to isovalent impurities (carbon, silicon and tin) in germanium. The A-center is an oxygen interstitial atom near a lattice vacancy and is an important impurity-defect pair in germanium. In germanium doped with carbon or silicon, we calculated that most of the binding energy of the cluster formed between A-centers and the carbon or silicon atoms is due to the interaction between the oxygen interstitial atom and the carbon or silicon atoms. For tin-doped germanium most of the binding energy is due to the interaction of the oversized tin atom and the lattice vacancy, which essentially provide space for tin to relax. The nearest neighbor carbon–oxygen interstitial and the silicon–oxygen interstitial pairs are significantly bound, whereas the tin–oxygen interstitial pairs are not. The results are discussed in view of analogous investigations in isovalently doped silicon.

Density functional study of carbon doping in ZnO

The formation energy and charge states of substitutional and interstitial C impurities and their complexes in ZnO have been studied using density functional theory calculations. While single CZn defects have the highest absolute stability, interstitial C in n-type ZnO prefers to form interstitial C2 pairs or CZn–Ci complexes, thereby lowering the defect formation energy. Moreover, those atomic C impurities that have low formation energy are found to be nonmagnetic in their stable charge states. However, both in p-type and n-type ZnO, certain charge states of C2 complexes possessing a spin magnetic moment are identified. This might give a clue why both p-type and n-type magnetism have been reported for C-doped ZnO samples.

Stability of Frenkel pairs in Si(1 0 0) surface in the presence of germanium and oxygen atoms

A first-principles pseudo-potential study of Frenkel pair generation close to the Si(1 0 0) surface in the presence of germanium and oxygen atoms is reported. The energies and structures of the defect structures (i.e. vacancy and relaxed tetrahedral Si interstitial) are calculated using supercell with up to 88 atoms. We present results obtained using the generalized gradient approximation (GGA) for the exchange–correlation energy. We examine the effect of the presence of germanium and oxygen atoms on the stability of Frenkel pairs generated near the Si(1 0 0) surface by comparing a number of individual cases, starting from vacancy interstitial pairs situated at various positions. The general tendency of the created interstitials is to climb towards the surface, but they generally remain in subsurface layers, ready to migrate into the layer. This tendency is enhanced by the presence of the Ge and/or O atoms. We show that the formation energy is lower and Si interstitials can be created with energies as low as 1.5 eV.

Interaction of copper impurity with radiation defects in silicon doped with boron

The spectrum of deep levels formed in boron-doped Czochralski-grown silicon single crystals as a result of interaction of radiation defects with copper impurity is studied. It is shown that, irrespective of the order of introduction of defects (both in the case of low-temperature copper diffusion into crystals preliminarily irradiated with electrons and in the case of irradiation of the samples contaminated with copper), the same set of deep levels appears. In addition to conventional radiation defects, three types of levels have been detected in the band gap of copper-containing crystals. These levels include the level E v + 0.49 eV (already mentioned in available publications), the level E v + 0.51 eV (previously not related to copper), and a level close to the donor level of a vacancy. Based on the analysis of concentration profiles, the interstitial carbonoxygen pair is excluded from possible precursors of the copper-containing center with level E v + 0.49 eV.

Characterization and transport properties of 10BaO–xAg2O–(85-x)V2O5–5TeO2 glass system

Silver-based quaternary glasses were prepared by splat quenching technique. X-ray diffraction and differential scanning calorimetry were done for confirming their amorphous nature. The conductivity of the glasses was measured in the frequency range from 1 Hz to 32 MHz from room temperature to 373 K. Conductivity data, which obeys the Arrhenius type behavior, shows minimum at 30 mol% Ag2O, suggesting that the conductivity mechanisms are different above and below these two regions. The minimum in conductivity is accompanied by an inverse behavior of activation energy. Experimental data suggests that a polaron hopping mechanism operates in the electronically conducting domain of 20 ≤ × ≤ 30, and an interstitial pair mechanism operates in the ionically conducting domain of 35 ≤ × ≤ 55.

Aggregation of interstitial copper atoms in silicon

The formation energy of metastable Cu aggregates consisting of one to four interstitial Cu atoms is examined using ab initio electronic structure theory. Localized vibration energies are calculated for two aggregates. These are a substitutional and interstitial Cu pair and three interstitial Cu atoms. In the latter aggregate there is extensive atomic relaxation with a Si atom moving into a nearby interstitial position and leaving a Si vacancy and Si interstitial. There is no bond breaking. The relaxation stabilizes the complex to have trigonal symmetry with an almost exact tetrahedral geometry. The results highlight that several Cu slightly metastable aggregates may be possible in silicon having strong localized modes at similar energies.

Temperature dependence of MeV-electron-irradiation-induced nanocrystallization in Zr–Pt metallic glass

MeV electron irradiation introduces density fluctuations in metallic glass, which lead to nanocrystallization at temperatures of 298 K and below in Zr 80Pt 20 metallic glass. This density fluctuation corresponds to the formation of a Frenkel pair, i.e., a vacancy–interstitial pair in the metallic crystal. Phase selection in MeV-electron-irradiation-induced crystallization exhibits temperature dependence. Two f.c.c. crystalline phases were reduced to one f.c.c. phase when the temperature was reduced from 298 K to 20 K. The formation of the quasicrystal (QC) phase during thermal crystallization affects MeV-electron-irradiation-induced crystallization.