Positron Lifetime Calculations Research Articles

First-principles calculations for intrinsic defects in MgO: Positron lifetime and momentum distribution of electron–positron pairs

We presented first-principles calculations of the formation energies for various charge states of intrinsic defects in magnesium oxide, including VO, VMg, VMg+O, VO+Mg+O, and VMg+O+Mg. These results can be used to predict their thermodynamic stability and detectability of these defects via positron annihilation spectroscopy. We employed two-component density functional theory to compute the positron annihilation properties, incorporating the effects of temperature and spin polarization. We then performed calculations of the momentum distributions of annihilating electron–positron pairs in these intrinsic defects in MgO. Using one-dimensional momentum distributions (Doppler-broadening spectra), we calculated the line-shape parameters Srel and Wrel. Our positron lifetime calculations were compared with experimental data. These research provides insight into the positron annihilation characteristics of defects in MgO.

The role of defect charge, crystal chemistry, and crystal structure on positron lifetimes of vacancies in oxides

Density functional theory based positron lifetime (PL) calculations for cation and oxygen monovacancies in a range of oxides-hematite, magnetite, hercynite, and alumina-have been conducted to compare the impact of defect chemistry and crystal structure on the predicted lifetimes. The role of defect charge state has also been examined. A comparison across the same type of crystalline structure but different composition shows that oxygen vacancies only induce a slight increase in the positron-electron overlap and thus barely modify the PL as compared to the bulk. A much more substantial increase of PL is observed for cation monovacancies, regardless of crystal structure or the elemental nature of the vacancy, which we ascribe to an enhanced localization of charge density around the vacant site. The structural and compositional richness of the oxide leads to longer defect PLs, with defected hercynite exhibiting the longest PLs. The charge state of cation monovacancies modifies only by a small percentage the positron localization, relegating to secondary importance the metal defect's oxidation state in modifying the lifetime of positrons within vacancy traps.

Calculation of positron lifetimes and momentum distributions of electron-positron annihilation radiation in Mg-doped 3C-SiC

We calculated the formation energies of various charge states for MgC, MgSi, MgSi+VC, and MgC+VSi defects in 3C-SiC to assess their thermodynamic stability. Using the two-component density functional theory, we determined the positron annihilation lifetimes at 0 K, temperature-dependent positron annihilation lifetimes while considering the electron-phonon coupling interactions, and the positron densities in various charge states of these defects in 3C-SiC. Additionally, the electron-positron momentum distributions were analyzed. Our calculated results largely agree with previous calculations and experimental data, providing a database for the identification of impurity-vacancy complexes in 3C-SiC.

Modeling and X-ray Analysis of Defect Nanoclusters Formation in B4C under Ion Irradiation.

In the presented work, B4C was irradiated with xenon swift heavy ions at the energy of 167 MeV. The irradiation of the substrate was done at room temperature to a fluence of 3.83 × 1014 ion/cm2. The samples were then analyzed with the X-ray diffraction technique to study the structural modification, as it can probe the region of penetration of xenon atoms due to the low atomic number of the two elements involved in the material under study. The nano-cluster formation under ion irradiation was observed. Positron lifetime (PLT) calculations of the secondary point defects forming nanoclusters and introduced into the B4C substrate by hydrogen and helium implantation were also carried out with the Multigrid instead of the K-spAce (MIKA) simulation package. The X-ray diffraction results confirmed that the sample was B4C and it had a rhombohedral crystal structure. The X-ray diffraction indicated an increase in the lattice parameter due to the Swift heavy ion (SHI) irradiation. In B12-CCC, the difference between τ with the saturation of H or He in the defect is nearly 20 ps. Under the same conditions with B11C-CBC, there is approximately twice the value for the same deviation.

Characterization of lattice distortions in refractory metal complex concentrated alloys using positron annihilation spectroscopy

Severe distortions of crystalline lattice represent an inherent feature of complex concentrated alloys (CCAs). Lattice distortions are caused by atomic size differences of alloy constituent ions which are distributed randomly over the lattice sites. It is believed that lattice distortions play the key role in extraordinary properties of CCAs. However, characterization of lattice distortions is difficult since they occur on the atomic scale and have a stochastic nature. In the present work we employ positron annihilation spectroscopy for characterization of lattice distortions in series of 18 refractory metal CCAs with the bcc structure. Our study revealed that the bulk positron lifetime increases with increasing magnitude of lattice distortions. It indicates that positron delocalized in the lattice can be used as a probe of lattice distortions in CCAs since it is located with higher probability in interstitial regions with extended open volume. This experimental observation is supported by theoretical calculations of positron lifetimes in CCAs. Moreover, it was confirmed that lattice distortions cause strengthening of CCAs.

Effect of Ag doping on crystallinity and microstrain of LaMnO3 nanoparticles: Confirmations of defect levels with positron lifetime and Doppler-broadening calculations

LaMnO3 nanoparticles with different levels of Ag doping are prepared and their impact on defect concentration and crystallite sizes are investigated. The crystallinity studies and phase identifications are done with XRD analysis. Williamson-Hall plots and size-strain plots are used to analyse the effect of doping on crystallinity and defect induced microstrain. With the help of Positron Annihilation spectroscopy, the identification of defects and mapping of the defect densities are carried out. Positron lifetime calculations and analysis on Doppler-broadened profile of the positron-electron annihilation radiation in undoped and various doped LaMnO3 are carried out and compared with W-H plots and size-strain plots.

Defects in H/He neutral beam irradiated potassium doped tungsten alloy by positron annihilation technique

Pure tungsten (W) is a primary plasm-facing material (PFM) candidate because of its superior properties, but it still has some drawbacks. In order to solve these problems, various methods have been used to improve the performances of tungsten-based materials. Potassium (K) doping, as one of the typical dispersion-strengthening methods for W materials, improves low temperature brittleness, reduces the ductile-brittle transition temperature, and suppresses the recrystallization. Meanwhile, it also improves the thermal shock resistance and fracture toughness of the material by introducing nano-sized K bubbles. However, this method brings a large number of defects inevitably. In fact, the K bubbles and the dislocations which are pinned by these K bubbles can affect the morphology and evolution of hydrogen (H) and helium (He) atoms in the alloys. As a very sensitive method to detect vacancy-type defects in materials, positron annihilation spectroscopy is used to study the irradiation damage caused by H and He atoms in this paper. The calculation of positron lifetime shows that positrons are more sensitive to the vacancy-type defects. Bounding of H and He with vacancies reduces the positron lifetime because of the increase of valence electron density. Combining the calculated results with the positron annihilation lifetime spectrum (PLAS) results shows that the W-K alloy is easier to promote the H atoms to release. Besides, it also more likely to form larger He bubbles which can be estimated by positron lifetime values. The result is also confirmed by the measurements from the scanning electron microscope (SEM) and slow positron Doppler broadening spectroscopy (DBS). The defects in the W-K alloy such as K bubbles and their pinned dislocations can act as diffusion channels to promote the H atoms to release, which gives rise to a smoother surface under the pure H irradiation. Meanwhile, under the condition of the H+6%He irradiation, the K bubbles and their pinned dislocations in the W-K alloy become the capture center of He atoms, promote their nucleation and grow into larger He bubbles. Moreover, under the action of stress and temperature gradient, some of the He bubbles migrate to the surface and release, this process is conducive to the recovery of related defects and the reduction of radiation damage.

Positron annihilation apectroscopy on colloidal CuIn1-xGaxSe2 semiconductor sanocrystals

Simple and low-cost hot-injection method was used to synthesize three samples of colloidal nanocrystals with general chemical formula CuIn1-xGaxSe2 (x=0.0, 0.6 and 0.82). X-ray diffraction (XRD) and transmittance electron microscopy (TEM) have been used to investigate the structural properties of the synthesized nanocrystals and proved their high crystallinity. Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques were used to give more insights on the structural defects of the grown samples. The results of the positron lifetime measurements indicate that the shortest lifetime component (τ1) with intensity ≳ 97% is greater than the calculated positron bulk lifetime, 235-240 ps. This indicates that the concentrations of the vacancy-type defects for the as-synthesized CuIn1-xGaxSe2 samples are greater than the saturation trapping limit (1018 cm-3). Moreover, results of the PAL measurements and the theoretically calculated positron lifetimes indicate that Cu-Se, In-Se and/or Ga-Se are the dominant vacancy-type defect for all studied range of Ga. Also, the results show that the vacancy-type defect concentrations of samples with Ga content of 0.6 and 0.82 are reduced compared with that for zero-Ga content sample.

Positron lifetimes of bare and hydrogenated zirconium vacancies in cubic yttria-stabilized zirconia: an ab initio study

Recent studies by positron-annihilation spectroscopy (PAS) in single-crystal and nanostructured yttria-stabilized zirconia (YSZ) revealed extensive positron trapping at vacancy-type point defects and other structural imperfections. The present work reports first principles calculations of formation energies and positron lifetimes of Zr vacancies in 10.3 mol% cubic YSZ. The lifetime calculations are based on two-component density-functional theory within the local-density approximation for the electron-positron correlation energy. Gradient-correction effects were also examined. Zr monovacancies were found to be potent trapping sites for positrons with large binding energies and lifetimes in excess of 220 ps. A strong dependency of the lifetimes on the Zr site and local coordination number was observed, a consequence of the structural disorder of the stabilized lattice. In contrast, oxygen vacancies did not act as attractive centers for positrons, either as single defects or as nearest neighbors to seven-fold coordinated Zr vacancies. Defect association of the Zr vacancies with hydrogen led to stable (-H) complexes with hydrogen bound to oxygen ions which are nearest neighbors to the vacancies. The negatively-charged (-H) defects were also found to trap positrons with corresponding lifetimes strongly dependent upon the hydrogen content within the complex.

Defects and their range in pure bismuth irradiated with swift Xe ions studied by positron annihilation techniques

Investigations of defects and their spatial distribution in Bi irradiated with 167 MeV Xe26+ ions of different doses have been performed using conventional positron lifetime spectroscopy and variable energy positron beam. In an implanted layer, in which ions are traveling, interacting with atoms and stopping, only clusters which consist of more than eight vacancies were found. It was assigned from ab initio theoretical calculations of positron lifetime in vacancy clusters in Bi. The thickness of this layer corresponds to the range of implanted ions calculated from the SRIM code. However, beyond this layer, an extended layer with such defects has also been found. Its thickness is comparable to the thickness of the implanted layer and it depends on the dose. Defects induced by implantation are also present near the entrance surface, and their concentration depends on the dose of implanted ions as well. Three methods for reconstructing the actual mean positron lifetime and thus the induced depth defect distribution have been proposed, two of them are used in current research.

Probing the role of cation vacancies on the ferromagnetism of La-doped BiFeO3 ceramics

Single-phase multiferroic Bi1−xLaxFeO3 (x = 0.00, 0.10, 0.20, 0.30, and 0.40) ceramics were studied by positron annihilation lifetime spectroscopy and the effect of the cation vacancies on the magnetism were investigated. Positron lifetime measurements and density-functional theory calculation of positron lifetimes identified the presence of Bi and Fe vacancies. It was observed that when x > 0.20 the Bi monovacancies disappeared gradually and some larger vacancy clusters begin to appear. Magnetic property measurements showed that the remnant magnetization increases greatly when x > 0.20. It is found that the enhanced magnetization is not only due to the suppressed spin cycloid structure, but the variation of cation defects also contributes.

Positron lifetime calculation of vacancy clusters in tantalum containing hydrogen and helium

Tantalum (Ta)-cladded tungsten (W) is predominantly used in a spallation target to obtain a high yield of neutrons. Investigation of changes in the mechanical properties and evolution of microstructures in Ta under irradiation of high-energy neutrons is essential to maintain the safety of the target of the spallation neutron source. In this study, the effects of helium (He) and hydrogen (H) on the positron lifetimes of vacancy clusters were examined. The calculation results indicate that the positron lifetime of perfect Ta is 118.3 ps and the positron lifetime is 216.5 ps when single vacancies exist. The positron lifetime increases with increase in the number of vacancies in the vacancy cluster. The positron lifetime of single vacancies decreases to 191.7 ps when the vacancy contains an H atom, whereas it decreases to 158.6 ps when the vacancy contains a He atom. This shows that the effect of He on positron lifetime change is larger than that of H.

Positron Lifetime Calculation for Plastic Deformed Nanocrystalline Copper

Positron lifetime calculation has been performed on a computer-generated nanocrystalline copper with a mean grain size of 9.1 nm during its deformation. For the undeformed and deformed nanocrystalline copper, calculated positron lifetimes are around 157 ps which come from the positron annihilation in the free volume in grain boundaries. Due to the grain-boundary deformation mechanism, no vacancies or vacancy clusters will be induced in grains during the plastic deformation of the nanocrystalline copper, which is different to the deformation of the conventional polycrystal. From this point of view, in-situ positron annihilation measurements can provide important experimental information on the deformation mechanism of nanocrystalline metals.

Defect Characterization in Thermoelectric Zn-Sb Systems by Positron Annihilation

Positron lifetimes and momentum distributions of annihilating electron-positron pairs have been calculated for vacancies in ZnSb and Zn4Sb3. The calculated positron lifetimes for bulk ZnSb and Zn4Sb3 are 203 ps and 208 ps, and for VZn in ZnSb and Zn4Sb3 are 249 ps and 237 ps respectively. The calculated momentum distribution results indicate the VZn in both ZnSb and Zn4Sb3 has less characterization from elemental Zn. Using coincidence Doppler broadening spectra combined with lifetime measurements can reveal the vacancy structure in ZnSb and Zn4Sb3.

Characterization of point defects in CdTe by positron annihilation spectroscopy

Positron lifetime measurements on CdTe 0.15% Zn-doped by weight are presented, trapping to monovacancy defects is observed. At low temperatures, localization at shallow binding energy positron traps dominates. To aid defect identification density functional theory, calculated positron lifetimes and momentum distributions are obtained using relaxed geometry configurations of the monovacancy defects and the Te antisite. These calculations provide evidence that combined positron lifetime and coincidence Doppler spectroscopy measurements have the capability to identify neutral or negative charge states of the monovacancies, the Te antisite, A-centers, and divacancy defects in CdTe.

Investigation and calculation of positron lifetimes of monovacancies in crystals

The first-principles calculations of positron lifetimes of mono-vacancies in crystals were investigated. We use the two-component density functional theory to respectively compute positron lifetimes of neutral charge state of VAl defect in aluminium, VSi defect in silicon, VC, VSi and VC+CSi defects in 3C silicon carbide, VGa and VAs defects in gallium arsenide, taking into account atomic relaxation due to vacancy and electronic structural relaxation due to the presence of the positron. Three different calculation schemes are used. We find that the electron density inside the vacancy more or less increases due to the presence of the positron if the ionic positions are kept fixed, and the positron becomes more localized after the electronic structural relaxation for the case of VAl defect in aluminium and VSi defect in 3C silicon carbide, but it is opposite for the case of VGa defect in gallium arsenide and VC defect in 3C silicon carbide. The results with no consideration of the relaxation are even much closer to the experimental ones, therefore the atomic relaxation due to the position play an important role in calculating the positron lifetime of mono-vacancies in crystals.

Positron lifetime calculation for possible defects in nanocrystalline copper

Structural models for dislocation, vacancy clusters, twin boundary, stacking fault and nanocrystalline sample are constructed using copper as a model material. Positron lifetimes and momentum distributions of annihilating electron–positron pairs are calculated for these structural models. The calculated results indicate that the dislocation, twin boundary and stacking fault are shallow traps to positrons. The dislocation associated with monovacancies gives rise to a positron lifetime similar to that of monovacancies. The calculated positron lifetimes of the nanocrystalline copper show no dependence on the mean grain size. The as-constructed nanocrystalline samples contain vacancy clusters in grain boundaries, and positrons are localized by the vacancy clusters. However after relaxation the samples show only other two kinds of free volumes: one is the interatomic space in grain boundaries which is a shallow trap to positrons; the other is similar to a monovacancy. The latter contributes a positron lifetime of about 163 ps. This kind of free volume is not only observed in grain boundaries but also in the regions near grain boundaries. Positron lifetime calculation combined with the momentum distribution calculation is useful to identify the defect in the nanocrystalline Cu.

Exploring positron characteristics utilizing two new positron–electron correlation schemes based on multiple electronic structure calculation methods**Project supported by the National Natural Science Foundation of China (Grant Nos. 11175171 and 11105139).

We make a gradient correction to a new local density approximation form of positron–electron correlation. The positron lifetimes and affinities are then probed by using these two approximation forms based on three electronic-structure calculation methods, including the full-potential linearized augmented plane wave (FLAPW) plus local orbitals approach, the atomic superposition (ATSUP) approach, and the projector augmented wave (PAW) approach. The differences between calculated lifetimes using the FLAPW and ATSUP methods are clearly interpreted in the view of positron and electron transfers. We further find that a well-implemented PAW method can give near-perfect agreement on both the positron lifetimes and affinities with the FLAPW method, and the competitiveness of the ATSUP method against the FLAPW/PAW method is reduced within the best calculations. By comparing with the experimental data, the new introduced gradient corrected correlation form is proved to be competitive for positron lifetime and affinity calculations.

Defect structure of thermoelectric Zn4Sb3

Thermoelectric samples prepared by spark plasma sintering method have been studied by resistivity measurement, X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS). The existence of Zn vacancies in structure is confirmed by PAS, which contribute a positron lifetime component of ps. Positron lifetime calculation has been performed for different defect structural models of . By comparing the calculated lifetime values with the experimental result, we find that each Zn vacancy must have at least two close interstitial Zn atoms in the crystal structure of . Two separate phase transitions observed by resistivity measurement occur at about 232 and 252 K, whereas the positron annihilation measurements indicate they are about 225 and 240 K. Positron lifetime and Doppler broadening results reveal phase is a transition phase between and phases. The results also indicate the disordering of interstitial clusters into interstitial Zn sites during the to phase transition. The interstitial Zn clusters are found to act as shallow positron traps which apparently capture positrons below 200 K.

Coupled experimental andDFT+Uinvestigation of positron lifetimes inUO2

We performed positron annihilation spectroscopy measurements on uranium dioxide irradiated with 45 MeV $\ensuremath{\alpha}$ particles. The positron lifetime was measured as a function of the temperature in the 15--300 K range. The experimental results were combined with electronic structure calculations of positron lifetimes of vacancies and vacancy clusters in ${\mathrm{UO}}_{2}$. Neutral and charged defects consisting of from one to six vacancies were studied computationally using the $\mathrm{DFT}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}U$ method to take into account strong correlations between the $5f$ electrons of uranium. The two-component density functional theory with two different fully self-consistent schemes was used to calculate the positron lifetimes. All defects were relaxed taking into account the forces due to the creation of defects and the positron localized in the vacancy. The interpretation of the experimental observations in the light of the DFT + $U$ results and the positron trapping model indicates that neutral ${\mathrm{V}}_{\text{U}}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}2{\mathrm{V}}_{\text{O}}$ trivacancies (bound Schottky defects) are the predominant defects detected in the 45 MeV $\ensuremath{\alpha}$ irradiated ${\mathrm{UO}}_{2}$ samples. Our results show that the coupling of a precise experimental work and calculations using carefully chosen assumptions is an effective method to bring further insight into the subject of irradiation induced defects in ${\mathrm{UO}}_{2}$.